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Author Topic: ie. 28mm 'normal' vs 28mm 'wide'  (Read 2939 times)
duncanbourne
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« on: April 11, 2013, 08:18:41 PM »
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Can a 28mm full-frame lens accurately be described as a 'normal' lens for APS-C purely by virtue of it's focal length or is it always going to be a 'wide angle lens that gives the equivalent coverage to a normal for that sensor format'?
Wondering if there are differences in lens design between 'wide', 'normal' and 'tele' lenses of the same focal length - I realise that they cover different image circles, but if we could somehow ignore that.
Am I making any sense?
Duncan
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marcmccalmont
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« Reply #1 on: April 11, 2013, 08:59:37 PM »
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For all practical purposes no there is no difference but, I hope the smart people here chime in, there is a difference between retrofocus, normal and telephoto designs. I'm sure if you google that you will find a good explanation of lens design. Changing sensor formats with a given lens will change the equivalent focal length and DOF.
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Marc McCalmont
theguywitha645d
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« Reply #2 on: April 11, 2013, 09:58:48 PM »
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Wide, normal, and telephoto usually refer to apparent perspective/field of view of a lens with a given format. Normal is a focal length equal to the diagonal of the format. It is "normal" in that the standard viewing distance (as defined as the distance equal to the image diagonal) and the correct viewing (distance as defined as a viewing distance proportional to the focal length) are equal. The result is an image that has an apparent perspective of an actual viewer (normal has nothing to do with the field of view of the human eye). Wide is for focal lengths shorter than normal and telephoto for focal lengths longer.

There are other optical definitions of wide and telephoto. Wide can mean a view camera lens that has a certain angular size, irrespective of the format. Telephoto can refer to a design whose focal length is longer than the physical length of the lens. But neither of these really speak to your question.

In terms of the image you would make, focal length is focal length. It will not matter for which format the lens was designed for.
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Petrus
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« Reply #3 on: April 11, 2013, 11:57:46 PM »
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If you have a 28mm lens designed for a full frame (135) camera attached to a APS-C sensor body, you have a wide-angle lens (by the construction) which acts like a "normal" lens on your camera (based on the view angle), with great part of its image circle going to waste.

So it is a wide-angle used as a normal... I do not see any problem with that, if you use a school bus to transport chickens it still is a school bus.
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scooby70
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« Reply #4 on: April 12, 2013, 07:35:09 AM »
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When using larger format lenses on smaller format cameras what you get is a crop cut from the centre of the image the lens provides. So, a 28mm lens when used on a (x1.5 crop) APS-C camera gives an effective FoV of (28 x 1.5=) 42mm. The only real change is in the DoF as you're using a 28mm lens and not a 42mm lens.

I used to use a 30mm f1.4 on my (APS-C) Canon 20D and I was very happy with it and I currently use a 25mm f0.95 and a 28mm f2.8 on my (Micro Four Thirds) Panasonic G1 and get good results.
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jrsforums
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« Reply #5 on: April 12, 2013, 09:45:49 AM »
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When using larger format lenses on smaller format cameras what you get is a crop cut from the centre of the image the lens provides. So, a 28mm lens when used on a (x1.5 crop) APS-C camera gives an effective FoV of (28 x 1.5=) 42mm. The only real change is in the DoF as you're using a 28mm lens and not a 42mm lens.

I used to use a 30mm f1.4 on my (APS-C) Canon 20D and I was very happy with it and I currently use a 25mm f0.95 and a 28mm f2.8 on my (Micro Four Thirds) Panasonic G1 and get good results.

However, the "effective" DOF will change due to the crop.  This is true whether the crop is done in camera or in post.

John
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John
scooby70
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« Reply #6 on: April 12, 2013, 05:09:44 PM »
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However, the "effective" DOF will change due to the crop.  This is true whether the crop is done in camera or in post.

John

Discussions about DoF never end well on internet forums  Grin but I'd argue that simply cropping the image doesn't alter the DoF, how could it when all cropping is is cutting the edges off, the DoF wont alter. It will however alter if you alter the image further by printing the cropped image as large as you would have printed the whole image or if you view it much closer than you'd have viewed the whole image as either scenario could make things in the scene that did look like they were in the DoF when they were small in view look like they are not when they look larger in view.

The difference between, for example, 25mm on a small format like MFT (x2 crop just to keep the numbers simple rather than the somewhat messy comparison with APS-C  Grin) and 50mm on 35mm full frame ends up about the same as the crop factor. So, you'd have to use a 25mm lens at a f1 on a small format camera to get about the same look as you'd get with a 50mm lens at f2 on a FF camera.

These days many of us have cameras with different sensor sizes so things like this are quite easy to check. The first of the following images was taken with a micro four thirds camera and a 25mm at f0.95 and the second image was taken with a 5D and 50mm at f2. The framing isn't exactly the same as I just shot these casually hand held but it's close enough and the DoF is close enough too, as it should be.

Of course you can get shallower DoF from smaller format cameras with equivalent focal length lenses by simply reducing the camera to subject distance. This will get you shallow DoF if you're willing to accept the tighter framing.





 Grin
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Telecaster
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« Reply #7 on: April 12, 2013, 05:19:34 PM »
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The assumption often inherent in questions of this sort is that certain focal lengths are intrinsically wide, others intrinsically normal and still others intrinsically long. This is most definitely not the case.

It all comes down to format size. If you shoot 4x5" sheet film your "normal" lens is a 150mm. If you shoot with the Micro-Four-Thirds system your normal is a 25mm. Etc.

We tend to relate all this stuff to 35mm due to its dominance during the 1950s thru early 2000s (and its continued dominance among pros even today). But there's nothing special about the 35mm format that causes lenses to reconfigure themselves when mounted on a 35mm camera.   Wink 

As for retrofocal and telephoto lens designs...there's nothing preventing a lens maker from producing a 28mm telephoto lens for a small-sensor system, or a 200mm retrofocal lens for a really large-sensor system.

-Dave-
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scooby70
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« Reply #8 on: April 12, 2013, 05:27:44 PM »
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The assumption often inherent in questions of this sort is that certain focal lengths are intrinsically wide, others intrinsically normal and still others intrinsically long. This is most definitely not the case.

Yes. The "look" you get from different lenses and format to format may well alter as you change your camera to subject distance, aperture, shutter speed and even ISO... but when looking at images from just one format it may be difficult to tell what lens they were taken with.

I use manual lenses a lot with my micro four thirds camera and there's no focal length or aperture in the exif and unless I can remember which lens I took the shot with I can't really tell and getting back to the OP's question, 28mm on a smaller format can indeed look like 50mm on a larger format.
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theguywitha645d
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« Reply #9 on: April 12, 2013, 06:44:46 PM »
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Discussions about DoF never end well on internet forums  Grin but I'd argue that simply cropping the image doesn't alter the DoF, how could it when all cropping is is cutting the edges off, the DoF wont alter.

DoF is dependent of format--whether you change format or crop does not matter. DoF does change with cropping because you are changing format. But you are not simply cutting the edges off in cropping. You are also effectively enlarging the image. Since you are also enlarging the CoC at the same time, the CoC that was acceptably sharp in the full image, can become too large in the crop.
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jrsforums
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« Reply #10 on: April 12, 2013, 07:15:40 PM »
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DoF is dependent of format--whether you change format or crop does not matter. DoF does change with cropping because you are changing format. But you are not simply cutting the edges off in cropping. You are also effectively enlarging the image. Since you are also enlarging the CoC at the same time, the CoC that was acceptably sharp in the full image, can become too large in the crop.

Thank you 645d

For reference: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

"...When does the circle of confusion become perceptible to our eyes? An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot..."

"...print size and viewing distance influence how large the circle of confusion appears to our eyes..."

As 645d said, if you crop, either in camera or post, you effect the acceptable CoC for a given print size.

A more important point that many forget about.  The DOF calculations are based on a 8x10 (or some a 5x7) print.  If you are planning on printing larger than that you may want to consider getting more DOF....or ensuring that the images are viewed at a greater distance.

John

Just a note...the OP question was same lens with different format, i.e. FF vs APC.  My initial assumptions were lens settings, distance, and print size are the same.  DOF calculations change when you vary the focal length, aperture, sensor size, print size, viewing distance, etc.  One can really muddy the water if some of the variables are not held constant.
« Last Edit: April 12, 2013, 07:23:00 PM by jrsforums » Logged

John
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« Reply #11 on: April 12, 2013, 07:28:24 PM »
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It will however alter if you alter the image further by printing the cropped image as large as you would have printed the whole image

That is how DOF calculations are done.  Underlying assumption is printed at a set size; viewed at a specific viewing distance; with specific (20/20) eyesight.
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John
scooby70
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« Reply #12 on: April 12, 2013, 09:18:04 PM »
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DoF is dependent of format--whether you change format or crop does not matter. DoF does change with cropping because you are changing format. But you are not simply cutting the edges off in cropping. You are also effectively enlarging the image. Since you are also enlarging the CoC at the same time, the CoC that was acceptably sharp in the full image, can become too large in the crop.

 Grin

Ok. and that is perfectly correct, but I don't see that in practice as I find that I can largely ignore the effect of CoC if I keep things... reasonable... If I keep the changes in format size reasonable and don't go to extremes (like comparing a compact to large format at the same larger image size) and keep the crop/degree of enlargement reasonable and don't go to extremes (so that I stay within the bounds of what each format can reasonably produce and retain some image quality, x2 doesn't seem too extreme) then the effect of the CoC on perceived DoF will be (arguably, if following my way of thinking) minimal and swamped by other things (like aperture and camera to subject distance and the relative size of the things in the frame) and cropping will be more like cutting the edges off  Grin

Here's a heavy crop from a 5D followed by the whole image. To me in the heavy crop the guy with the red bobble hat looks to be the only one in the DoF, the guy to the front looks almost in but not quite and the guys to the rear much less so. In the whole image things to my eyes are unchanged. The DoF should be less in the crop and more in the whole image but keeping crop / enlargement reasonable they look very similar to me.

The final two images are 5D and MFT, both 20mm this time. They're very similar even though the degree of cropping / enlargement varies, but I've kept the degrees reasonable, not too extreme.









And BTW, the wallpaper wasn't yellowing, it's an effect of the lighting and all looks well to the human eye!  Grin

After playing with my 5D, 20D and MFT kit I decided that a x2 crop isn't too much and that with lenses that aren't too exotic I can get more or less the look I can get from a 5D+50mm from an APS-C+30mm or MFT+25/28mm as long as I don't go to the extremes of enlargement. I've never printed larger than A3 and I don't tend to do heavy crops.

I've only once ever had to worry about the CoC and that wasn't in photography but when producing artwork for a play that needed to be seen by people at the back. In my photography and when producing images to a maximum of A3 with only light cropping I find that for me personally all worries about the CoC can be forgotten when using any of the DSLR's or CSC I've owned.

Anyway, that's my contribution to the question and I hope all that helped someone  Grin
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scooby70
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« Reply #13 on: April 12, 2013, 09:27:26 PM »
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PS. I forgot to add... Many people don't even print images theses days, they just view them on screen and although this can affect the perceived DoF I personally have found that (again, when keeping things reasonable) except at the very extremes of shallow DoF (when keeping the same framing) the smaller format APS-C and MFT can get a good way towards producing near enough the same image as a 35mm FF. And you can always reduce the DoF with smaller formats by reducing the camera to subject distance  Grin
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jrsforums
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« Reply #14 on: April 12, 2013, 10:39:24 PM »
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Scooby, if you are happy great...!!

I also never worry nor think about CoC.  I just think about practical implications. Printing larger or cropping significantly requires sharper images.  "In focus", as I am sure you are aware, is only one plane (or curve if field curvature comes into play)....everything else is some degree of out of focus.  The closer you look and/or the bigger you make it, the more this can be seen and you need to adjust for it....basic common sense works fine here.

Your display images are poor vehicles to show the image, they just don't have the resolution, clarity, or quality to show the problem...not your images...the vehicle.  I have lots of images that I have no embarrassment to show on my site, but would never print at 12"x18".

To a large extent, the images posted and viewed on screens are not much different from the film which was developed and printed at the corner drug store.  People are elated to just be able to recognize the person or location...to remember or show there friends, then put in a "drawer".  What we view as quality does not matter to them...99.9% of the time.  The work i put in my pictures is much more than my wife or family would expect...but, I do it for me, so I can be proud of it...and ya know...they do appreciate it.

Many say printing is dying....and it probably is, as most things do.  That does not mean stiving for quality is....we just do not have the replacement vehicle yet.

What will those just OK viewed images look like is a couple years when 4K and 8K, or larger, displays become common and the picture frame of the future.  If I am still here, my printed images will convert well to the new media.  Will the iPhoneography?
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John
scooby70
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« Reply #15 on: April 13, 2013, 07:15:50 AM »
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Your display images are poor vehicles to show the image, they just don't have the resolution, clarity, or quality to show the problem...not your images...the vehicle.  I have lots of images that I have no embarrassment to show on my site, but would never print at 12"x18".

Yeah, they're low res and some are at too high an ISO really and that doesn't do much for the quality and ultimate quality was never really a part of shooting the Southpark characters, I was just trying to see how close to FF look and DoF I could get with a smaller format. I just went through this when going to FF DSLR after years of shooting with, and getting used to, APS-C (after years of 35mm film.) I managed to convince myself that I could get pretty close to FF quality and DoF at reasonable print sizes with reasonable crops with APS-C or MFT if I stayed at low to midding ISO's and didn't expect to get (for example) FF 50mm f1.4 DoF at 25/30mm and f1.4 with a smaller format and with the same framing. Even with a 25mm f0.95 on MFT I can't quite match FF 50mm f1.4 DoF with the same framing but I can get close to anything over f2.5 or so.

The way this relats to the OP is that I think you can use a 28mm on APS-C and get pretty close to what you'd get with a 50mm on FF except the thin DoF at the widest aprtures. The way to try and match the DoF would be to live with the different framing and reduce the camera to subject distace.

Another thing that I didn't bring into this as it doesn't apply to the OP or to me as I mostly use legacy lenses with my MFT camera is that some of the smaller format lenses are sharp, sharper than some of their FF equiv. lenses so that increased sharpness may help to offset the problems caused by additional enlargement.
« Last Edit: April 13, 2013, 07:19:34 AM by scooby70 » Logged
duncanbourne
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« Reply #16 on: May 16, 2013, 08:02:25 PM »
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Thanks for all the responses folks, I think guywith a645d came closest to what I'm getting at, difference in optical designs.
 
I understand it's possible for a lenses optical design to be 'wide' or 'normal' or 'telephoto'.

So here's a rephrase of the question - if I were to have two hypothetical lenses of identical focal length, say 35mm, one with a 'wide' optical design and one with a 'telephoto' optical design, both shot on the same camera at the same aperture of the same subject and both able to cover my image circle would I see any difference in the images produced?

I completely understand that changing film/sensor format allows a lens to perform as wide, normal or tele but does a lens have visible 'wide' or 'telephoto' characteristics independent of the film/sensor size?

Duncan
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BJL
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« Reply #17 on: May 16, 2013, 08:48:41 PM »
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Can a 28mm full-frame lens accurately be described as a 'normal' lens for APS-C purely by virtue of it's focal length ...
Wondering if there are differences in lens design between 'wide', 'normal' and 'tele' lenses of the same focal length - I realise that they cover different image circles, but if we could somehow ignore that.
Yes and yes.

The description "normal" fora lens is usually just a reference to giving photographs that cover an angular field of view of about 50 degrees, or what you get when the focal length is about equal to the diagonal length of the image recorded on the sensor. So any lens of 28mm focal length used on a sensor of about 28mm diagonal size gives you a "normal" FOV.

But the design of a 28mm focal length lens for a larger format like 35mm (42mm diagonal) is different and more difficult than a 28mm lens designed as a "normal" for a format of about 28mm diagonal: the former requires a wide angle optical design to give good image quality over the larger image circle needed for the larger format. Another design challenge is that if designed for an SLR, the larger mirror assembly of the larger format requires the rear-most lens elements to be further from the focal plane than in a lens designed for the smaller format. In modern designs, that usually pushes the 28mm lens for 35mm format to something like an inverse telephoto design; almost certainly it will be bulkier than a design optimized for the smaller format.

But to the long detour in this thread about DOF: any 28mm lens used with the same "APS-C" sized sensor will give the same DOF at the same f-stop, so as ar as your questions are concerned, you can ignore that red herring debate about DOF and format size!
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Petrus
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« Reply #18 on: May 17, 2013, 12:39:24 AM »
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I understand it's possible for a lenses optical design to be 'wide' or 'normal' or 'telephoto'.

So here's a rephrase of the question - if I were to have two hypothetical lenses of identical focal length, say 35mm, one with a 'wide' optical design and one with a 'telephoto' optical design, both shot on the same camera at the same aperture of the same subject and both able to cover my image circle would I see any difference in the images produced?

I completely understand that changing film/sensor format allows a lens to perform as wide, normal or tele but does a lens have visible 'wide' or 'telephoto' characteristics independent of the film/sensor size?

Short answer: no.

Long answer: there might be, especially with digital sensors. Often the wide-angle lenses are designed as inverted telephotos, which means they are further away from the sensor surface than a symmetric design with the same focal length would be. This was done in SLR lenses to give enough space for the mirror. Now with digital it is necessary to make the light rays hitting the sensor corners more perpendicular. Many of us have noticed that using good quality wide-angle legacy rangefinder lenses in cameras like Fuji X-Pro1 is not a good idea, as they sit too close to the sensor and the corner quality suffers. Thus an inverted telephoto design is better, even without the mirror box. With normal lenses there is not this problem, and long focal length lenses are usually (always) designed as telephotos*, which actually means a design where the lens is made physically shorter than the focal length. That is ok as the last lens surface is far enough away from the sensor/film anyway.

So: if we have an inverted telephoto (telephoto meaning optical design, not focal length) wide-angle, "normal lens", maybe symmetrical, wide-angle, and a telephoto design wide-angle (referring to the sensor diagonal ratio), it is most likely that the inverted telephoto design would be furthest away from the sensor giving the best ray angles, "normal" design being closer and "telephoto" design closest, causing imaging problems with both sharpness and vignetting in the corners.

*) "telephoto" actually and strictly speaking refers to the optical design, not long focal length, even though it is now used to mean "long focal length lens" 99.99% of the time.
« Last Edit: May 17, 2013, 12:40:59 AM by Petrus » Logged
Fine_Art
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« Reply #19 on: May 17, 2013, 10:48:16 AM »
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Wide, normal, and telephoto usually refer to apparent perspective/field of view of a lens with a given format. Normal is a focal length equal to the diagonal of the format. It is "normal" in that the standard viewing distance (as defined as the distance equal to the image diagonal) and the correct viewing (distance as defined as a viewing distance proportional to the focal length) are equal. The result is an image that has an apparent perspective of an actual viewer (normal has nothing to do with the field of view of the human eye). Wide is for focal lengths shorter than normal and telephoto for focal lengths longer.


Well said.

32-35mm on APS-C would be the equivalent of a 50mm FF. The biggest difference IMO is wide angle lenses are inherently weaker than normals. The light has to go through more distortions of many elements. Corners often have chromatic aberration and/or other issues. Barrel/pincushion is usually greater.

You have an advantage taking the scene with a larger format.   
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