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Old Sep 14, 2006, 12:56 PM   #1
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I understand the reason a 50 will look like a 75 on a dSLR, relative to image size....my question is....will a 50 on a dSLR "tend to flatten" the image the same as a 75 on a film SLR? I have a 105 macro (or something like that), that I always like the portraits from it on my old film SLR...(nice perspective, no long noses, etc). Will a 60 or so lens on my Nikon dSLR have a similar effect or does the perspective effect stay with the focal length.

Hope I have asked this question in an understandable way....(I'm not convinced that I have!)
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Old Sep 14, 2006, 7:04 PM   #2
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I believe, but I'm not positive, that the effects of more telephoto (like flatening the image) will not be the same with the shorter lens and a smaller sensor.

I believe the effect you're seeing is an effect of the lens' focal length (and maybe subject distance) and therefor has nothing to do with the sensor size.

But this is one of those situations there "I know just enough to confuse myself" comes into play. I *think* I'm right, but I could just not know enough to answer it correctly.

Eric
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Old Sep 14, 2006, 8:58 PM   #3
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I'm in the same boat as you, probably with a little less "freeboard",.....Thanks for the response, my instincts are the same as yours on this.
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Old Sep 14, 2006, 9:16 PM   #4
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Eric is correct. The camera to subject distance is what gives you "compression", not sensor size, as long as the lenses on the two cameras are "equal"-ie the aps cam has a 100 mm lens while the full frame has a lens equal to the "crop factor of the aps. I'm no expert, so I did some googling and found this fairly fast=http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00D4so

Hope this helps- it did me:lol:

Robert
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Old Sep 17, 2006, 11:50 AM   #5
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To understand this you have to understand the vexed problem of cropping. If you have a Canon 5d camera, which is as you know a full frame sensor, fitting a 50 mm lens with its angle of view of 46 degrees the full image of 36 * 24 will be on the sensor.If you put the same lens on to a 10d camera where the sizeof sensoris 22.7 * 15.1. The lens is still 46 degrees AOV and the full circular image of 43 mm diameter will be on the focal plane of the body. But with the smaller sensor a lot of the image will miss the sensor. This will seem as tho the lens has turned intothe 30 degree AOV of a longer focal length lens So we have a smaller AOV and a smaller overall image on the sensor. When shown on the monitor this image has to be enlarged greater than the full frame image. giving the effect of a telephoto lens. DOF will be the same on both shots as this a condition of the lens and hyperfocal distance. If you have a calculator with the TRIG function on, it is easy to calculate the AOV for a lens with a different size sensor. But the true AOV for the lens stays the same.
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Old Sep 19, 2006, 10:20 AM   #6
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OK....so....back to square one! Based on your very clear description of how the image is really the same, it's only that the outter bits of it are not being recorded on the smaller sensor, I would say that any flattening, compression, perspective (or whatever we choose to call it) would be the same, too. Right?

In other words, the relative appearance (ie. big nose, small ears exaggeration) would be the same for any given focal length lens.

In still more words, If I took a picture of a person with a film camera with a 50mm lens, then the same person, same pose, from the same distance, with a small-sensor camera with the same lens, then enlarged both images to the same virtual size, they would look identical.

Apparent perspective compression would only occur if the two photos describe above were taken from different distances, making the resulting image appear the same size in the camera!

So....do you all think this is accurate?

Thanks.
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Old Sep 19, 2006, 10:37 AM   #7
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Perspective has nothing to do with the lens, its based on how far or close you are to the subject.

However, if you put a wide angle lens on your likely to walk right up to 12" away from thier face, while if you mount a 135mm lens you will probably find yourself backing up across the room. Thats why it seems like focal length has an effect, your ignoring the fact that your walking around.

Given that peoples heads are roughly the same size, and assuming you use a similar composition, any lens/camera combination that gives your the same Field Of View (angle, like 47° for a "normal" lens) will cause you to walk to the same spot when composing.

If you liked 105mm on 35mm film, you need a 70mm lens on digital to get the same FOV. This will cause you to stand in the same place, and therefore get the same perspective.

You could stand in that same spot (lets just call it ten feet from the subject) and use a super-wide lens, then crop off the sides of your image to get the same composition, and it will have the same perspective. (Obviously, you'll lose resolution doing this, which is why you'll prefer to compose it right in the camera using a proper focal length).


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