Sensor crop factor and macro photography

[Rubio]

At a given focal length, the lens crop factor describes the angle of view on, say, an APS-C sized sensor compared with the angle of view on a full-sized (35mm equivalent) sensor. Thus a 50mm lens on an APS-C camera with a 1.5x crop factor gives the equivalent field of view and apparent telephoto effect of a 75mm lens on a full-framed camera. This has to do with the size of the image circle on the focal plane. Understanding this, I have two questions.
First, does a macro lens designed to have a 1:1 magnification ratio on a full-frame camera at the same focal distance have a greater than life-size magnification ratio on the smaller sensor? It seems logical that it should, and I suppose I could just go to B and H to try a lens, but where's the fun in that?
My second question has to do with depth of field. Telephoto lenses by their nature tend to compress space, but is that apparent compression also related to sensor size? In other words, does a 50mm lens give the same apparent depth of field/compression of space on full and smaller-sized sensors, or will it show the same compression on an APS-C sensor as a 75mm lens will on a full-sized sensor given the same aperture and distance from the subject?

[anton_reno]

Even if your lens is designed specifically for 35 mm., a 1:1 macro lens should give you a 1:1 ratio on an APS sensor. Only the angle of view will change. For any given focal length lens, the DOF should be the same for any given scene and subject distance. A 50 mm. lens is just that: a 50 mm. lens, regardless of the sensor it is designed for or used with.

As for your statement, "I suppose I could just go to B and H to try a lens, but where's the fun in that," I can't believe you don't appreciate your good fortune to have such a toy store in your neighborhood. I've been to B&H twice and to me it was better than a trip to Disneyland. In Reno we have only one real camera store and they don't carry Sony or Minolta. I bought my A300 at Best Buy, but they are a very poor second choice for camera equipment, so I have to read a lot of fine print before I commit money to buy a body or lens. And I can't try it the camera or lens beforehand unless I want to go to Sacramento or San Francisco, a 300 or 500 mile round trip.

Anton

[BobF]

At a given focal length, the lens crop factor describes the angle of view on, say, an APS-C sized sensor compared with the angle of view on a full-sized (35mm equivalent) sensor. Thus a 50mm lens on an APS-C camera with a 1.5x crop factor gives the equivalent field of view and apparent telephoto effect of a 75mm lens on a full-framed camera. This has to do with the size of the image circle on the focal plane. Understanding this, I have two questions.
First, does a macro lens designed to have a 1:1 magnification ratio on a full-frame camera at the same focal distance have a greater than life-size magnification ratio on the smaller sensor?

No. The physics of optics don't change. A 50mm lens is a 50mm lens is a 50mm lens. Doesn't matter if it's on a large format, full frame SLR or cropped frame SLR. Magnification is the same at any given distance. That said, macro lenses are designed such that the maximum magnification (i.e., 1:1) is only achieved at the close focusing distance. So, if you have to move back from the subject to get the same image framed in the viewfinder, you lose 1:1.

It seems logical that it should, and I suppose I could just go to B and H to try a lens, but where's the fun in that?

What, you don't enjoy going to the biggest toy store in the world? :-p

My second question has to do with depth of field. Telephoto lenses by their nature tend to compress space, but is that apparent compression also related to sensor size? In other words, does a 50mm lens give the same apparent depth of field/compression of space on full and smaller-sized sensors, or will it show the same compression on an APS-C sensor as a 75mm lens will on a full-sized sensor given the same aperture and distance from the subject?

No. Again, the physics of optics don't change. Depth of field is a product of two things - magnification and aperture. Higher magnification = less DoF. The confusion lies with the standard for measuring DoF. Most people talk of DoF at the image plane. The standard; however, for measuring it is an 8x10 print. What does that mean? It means that enlargement plays a part. Enlargement is magnification, in essence. Here's the deal. If you've got a 50mm lens on a full frame DSLR you'll get a scene in view from, say, 20 feet. On a 1.5X APS-C camera you need to move back to 30 feet to get the same scene in view because of the narrower angle of view. Moving back with the same focal length (or zooming out with a zoom lens) means less magnification at the image plane. As a result, DoF at the image plane will be greater on the APS-C image. But when that smaller image is printed, it has to be 'enlarged' more than the full frame image. That additional 'enlargement' in printing offsets the image plane difference and the two 8x10 prints will have, for all intents and purposes, the same DoF. What this also means is that you'll basically get back your 1:1 with the additional print enlargement from the smaller sensor.

[langone_peter]

I disagree, the properties of the 'sensor/film' don't matter and where a 2cm object falls on the sensor is besides the point. Mentioning that a 2cm subject may only be partially on the sensor/film belabors the point, so assuming the subject we're talking about is entirely on the sensor/film makes the answer more clear. Weather or not all of or part of a subject makes it onto the sensor/film has no impact on the ratio that it was recorded at and therefore is irrelevant.

Regards,
Peter Langone

[BobF]

Maybe I'm missing something but I don't see any previous mention of a 2cm subject. Regardless, with a smaller sensor, if the desire is to get the entire subject in the frame then either you have to move further away, use a shorter focal length or some combination of the two. The net result is, in terms of macro photography, you lose 1:1.

[suci]

As usual Bob is right. Peter, it seems you are agreeing with Bob at least partly, but overall I have no idea what you said.

[ChanTran]

It should be noted that DOF and the compression of space or perpective is 2 different things. As Bob has said about the issue of DOF I need no further say anything about it. Compression of space as the OP said it, normally seen in shots with long lenses, isn't a property of the lens but rather related to camera to subject distance. So If with the same 50mm lens when mounted on the APS camera makes you take picture at longer distance from your subject than with a FF camera then yes you will see more of this compression effect.

[BobF]

Not quite, Chan. As you rightly said, perspective is a function of distance to subject. That's it. If you stand in the same place and take a shot with a 15mm lens and a 200mm lens, the perspective will be the same.

If; however, you take a shot with a 50mm lens on a full frame camera from 10 feet but have to back up to 15 feet to get the same image framed with a 50mm lens on a cropped frame camera, the perspective of those two images will be different. But.... if you simply used a shorter focal length from the same 10 ft distance, the perspective would be the same.

[ChanTran]

We actually said the same thing Bob! I said that if using the same 50mm lens on the aps camera MAKES you take pictures from longer distance then you will see the compression effect. Because the lens on the APS camera has narrower view, users TEND to take pics from a further distance.

[BobF]

OK, then I'm very confused because I don't see that we're saying the same thing.