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Author Topic: What happens to lens perspective on crop sensor?  (Read 3766 times)
MikeMike
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« on: April 26, 2010, 12:20:02 AM »
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Hello everyone,

I am looking to get either the 35mm or 50mm canon lens. I am using the Canon 20d, and am curious if when I am taking into account the 1.6 magnification, would the 50mm lens will look the same as an 80mm would on a full frame, in terms of perspective? or do I simply take into account the magnification properties of my sensor.

Thank you,
Michael
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Ray
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« Reply #1 on: April 26, 2010, 12:57:23 AM »
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Quote from: MikeMike
Hello everyone,

I am looking to get either the 35mm or 50mm canon lens. I am using the Canon 20d, and am curious if when I am taking into account the 1.6 magnification, would the 50mm lens will look the same as an 80mm would on a full frame, in terms of perspective? or do I simply take into account the magnification properties of my sensor.

Thank you,
Michael

There's a common confusion here. Many of us are aware of the perspective distortion that takes place when we use a wide-angle lens from close up (big nose in portraits etc), so we tend to associate such distortion with the focal length of the lens when we should really associate it with the distance from the subject to the lens.

A shot using a 50mm lens on a 20D should look approximately the same as the same scene shot on full frame with an 80mm lens, provided the distance to subject is the same.

What may differ are such factors as resolution and DoF. The f stop will have to be adjusted to maintain equal DoF in both shots. The 80mm lens on FF 35mm will generally need to be stopped down at least a stop or two, depending on distance to subject. My own tests tend to indicate if the subject being photographed is close, say a couple of metres or less, you'll probably need to stop down a full 2 stops with full frame. If the subject is more distant, then the 1.6x multiplier applied to F stop number should be about right.
« Last Edit: April 26, 2010, 01:01:44 AM by Ray » Logged
MikeMike
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« Reply #2 on: April 28, 2010, 02:11:04 PM »
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Thanks Ray,

Reason I'm asking is because my quickest lens is an f4. I'm thinking about getting the 35mm Canon lens for its speed and light weight, and was curious about how it would appear on my 20D.

I understand what you have said about shooting distance, though in my experience wider lenses have a fisheye effect (albeit minimal on a 35mm lens) on subjects even at a distance. Would the effect be that of a cropped 35mm lens's photograph? or do the properties of the optics somehow shift.

Thanks in advance,
Michael
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feppe
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« Reply #3 on: April 28, 2010, 03:30:32 PM »
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Ray is correct. Another important point to note is that you will change the distance to the subject with different lenses. You'll get closer to the subject when shooting portraits with a 35mm to fill the frame than with a 50mm. This is the reason why you (and everyone else) sees distortions with wider angles.

You are also right in that many wide angle lenses have all kinds of weird distortions, especially the cheaper ones, which exacerbate the problem.

If you're looking to shoot portraits, a 35mm on a crop sensor shows unacceptable (to me) distortion when filling the frame with the subject's head, and even 50mm is marginal for facials. I use 85mm for portraits on my 450D/550D.
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k bennett
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« Reply #4 on: April 28, 2010, 05:50:06 PM »
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For portraits, on a crop sensor camera like the 20D, I'll agree with feppe and say that while I like the 50mm, the 85/1.8 is the lens I use more often.

For a general purpose carry around lens, I like the 28/1.8. The 35/2.0 is a little long for a GP lens, and too short for a portrait lens.
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MikeMike
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« Reply #5 on: April 28, 2010, 06:12:01 PM »
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Good info.

I'm making my way to Israel later this summer, and want a general purpose lens. My only lens is a 24-105, which after a day of hiking in 40+ degree weather... well I might as well be carrying a brick.

What I need to do is play around with the lens I have now at the different focal lengths, but it's difficult to be satisfied when I have so many options in between. I remember the first time I used a fixed lens on my Pentax; at first I wanted to break it, but after a while it all became natural. Anywise, enough rambling. I don't intend to use this lens for portraits, mainly general shooting, and low light (prime lens speed is why I have leaned away from G11).

I think I've got what I need, but please chime in if you have something to add. All is well appreciated!

Michael
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PeterAit
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« Reply #6 on: April 28, 2010, 06:34:20 PM »
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Quote from: MikeMike
Thanks Ray,

Reason I'm asking is because my quickest lens is an f4. I'm thinking about getting the 35mm Canon lens for its speed and light weight, and was curious about how it would appear on my 20D.

I understand what you have said about shooting distance, though in my experience wider lenses have a fisheye effect (albeit minimal on a 35mm lens) on subjects even at a distance. Would the effect be that of a cropped 35mm lens's photograph? or do the properties of the optics somehow shift.

Thanks in advance,
Michael

Think of a photograph taken with a given lens on a full-frame sensor and printed full frame. Then cut off the edges of the print to leave the area that would be covered by your smaller sensor. Did the perspective change? No.
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Peter
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MikeMike
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« Reply #7 on: April 28, 2010, 07:21:49 PM »
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Quote from: PeterAit
Think of a photograph taken with a given lens on a full-frame sensor and printed full frame. Then cut off the edges of the print to leave the area that would be covered by your smaller sensor. Did the perspective change? No.


Very clear. Thank you
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k bennett
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« Reply #8 on: April 28, 2010, 08:13:32 PM »
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Quote from: MikeMike
What I need to do is play around with the lens I have now at the different focal lengths, but it's difficult to be satisfied when I have so many options in between.


Easy solution. Pick a focal length you want to try for a while. Get some gaffer's tape -- not duct tape -- and tape the zoom ring on your 24-105 to that focal length. It won't be quite the same -- the fixed lenses have wider maximum apertures, so you won't get quite the same look -- but it will help you narrow down your choices.

When you want to try a different "lens" just re-tape the zoom ring.
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elf
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« Reply #9 on: April 29, 2010, 02:13:44 AM »
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Quote from: MikeMike
Good info.

I'm making my way to Israel later this summer, and want a general purpose lens. My only lens is a 24-105, which after a day of hiking in 40+ degree weather... well I might as well be carrying a brick.

What I need to do is play around with the lens I have now at the different focal lengths, but it's difficult to be satisfied when I have so many options in between. I remember the first time I used a fixed lens on my Pentax; at first I wanted to break it, but after a while it all became natural. Anywise, enough rambling. I don't intend to use this lens for portraits, mainly general shooting, and low light (prime lens speed is why I have leaned away from G11).

I think I've got what I need, but please chime in if you have something to add. All is well appreciated!

Michael

I'd get the 50mm f1.8 and shoot panoramas when you want a wider FOV.  With a little practice you should be able to shoot small handheld panos nearly as fast as shooting single shots. As others have said the perspective of an image is based on distance (and angle) between the camera and the subject, not on the focal length of the lens.  When stitching panoramas, you choose the focal length based on the amount of detail desired.
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MikeMike
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« Reply #10 on: April 29, 2010, 01:52:46 PM »
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Quote from: elf
I'd get the 50mm f1.8 and shoot panoramas when you want a wider FOV.  With a little practice you should be able to shoot small handheld panos nearly as fast as shooting single shots. As others have said the perspective of an image is based on distance (and angle) between the camera and the subject, not on the focal length of the lens.  When stitching panoramas, you choose the focal length based on the amount of detail desired.


Thats a pretty cool idea!

I still think 50mm will be too long though. I'm bringing one lens, and that lens has to be functional in both the desert and in the bars.

All the best,
MIchael
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ErikKaffehr
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« Reply #11 on: April 29, 2010, 02:56:42 PM »
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Hi,

If you have the same vantage point you would have nearly identical results on full frame with an 80 mm lens as with a 1.6X sensor with a 50 mm lens. Depth of Field will be slightly better on the 1.6X sensor if same aperture is used.

Best regards
Erik



Quote from: MikeMike
Hello everyone,

I am looking to get either the 35mm or 50mm canon lens. I am using the Canon 20d, and am curious if when I am taking into account the 1.6 magnification, would the 50mm lens will look the same as an 80mm would on a full frame, in terms of perspective? or do I simply take into account the magnification properties of my sensor.

Thank you,
Michael
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