Originally posted by Ben_Editct Why would the aperture be transformed?
Marc has already explained this well. Thanks Marc.
For those that remain unconvinced, try reading
the wikipedia article on DOF vs format size.
Basically, DOF is controlled by the aperture diameter. If you change the focal length (in order to obtain the same FOV on different formats) you also need to change the f-ratio in order to get the same aperture diameter again (as the f-ratio is defined by focal-length/aperture-diameter).
Originally posted by wallyb Further, you have the same DOF in a 40/2.8 lens on FF as you do in a 40/2.8 on APS-C, it's just that the image has been cropped.
We are talking about "equivalent images", i.e., how to take an image on a crop camera that is identical to one taken on an FF camera. If you want to do this, you need to adjust more than just the focal length.
Originally posted by wallyb You will never get the same image, because the sensor size is different.
You can compensate for the different sensor size by adjusting the focal length to get the the same FOV, f-ratio to get the same DOF, ISO to get the same shutter speed. The maths is not difficult and has been done many times.
Originally posted by Marc Sabatella Maybe if your expectation of what kind of aperture you might need is based on prior experience with film, where ISO 400 was really pushing your luck.
I've never used a film SLR. I find the term "fast lens" a bit unfortunate nowadays because I see the advantages of wide aperture lenses in the DOF control they offer. Yes, they also let more light in, but as you say with modern DSLRs shooting in low light can often more easily be had with higher ISO settings and in contrast to using wide apertures, one still has usable DOF.
Originally posted by Marc Sabatella ...you pretty much neutralize the inherent noise advantage of the larger sensor.
As it becomes clear from your later statements, a larger sensor has no noise advantage (it has a dynamic range advantage; the noise advantage only comes into play when one is able to use FF lenses whose APS-C equivalences do not exist, such as a 85/1.4). Anyone arguing this please check out this thread "
low-noise-benefit-ff-vs-aps-c-equals-zero".
Originally posted by Marc Sabatella But it does break down a little when you try making appeals like the above, because while a 60/4 sounds silly slow, that's mostly based on our collective experience with film, or experience we've had from reading about film.
An equivalence is an equivalence. It doesn't break down for some cases. In order to get an image with an APS-C (say 1.5 crop) sensor that is equivalent to one made with a 60/4 lens using an FF sensor, you need a 40/2.67 lens. Make the FF lens a 60/2.8 and your APS-C lens needs to be a 40/1.86.
Originally posted by Marc Sabatella Sort of like trying to depress who is already concerned about about the weight by telling them, "yeah, not only do you weigh 200 pounds, but that's 90718 grams".
I see it as saying to someone "Yes, you can state your weight is 0.091 tons, but that doesn't make you lighter my dear. Your weight is still 91kg."
We are basically saying the same thing, except that I believe that many people mistake an APS-C f/2.8 with the FF f/2.8, i.e., think they can get less DOF than they can really get.
Scale this down to P&S sensor formats and then suddenly pretty much everyone knows how difficult it is getting thin DOF on such cameras even though the lenses often are specified as f/2.8 as well. Their FF equivalent f-ratio is much higher.
Originally posted by Damn Brit It's even simpler to not make comparisons in the first place and just take pictures instead.
Why didn't you take a picture instead of making this comment?