Camera Labs

Hello Gordon,
I've enjoyed reading your reviews. Being a Canon fan (G3 and 350) I've read the Canon lens comparison as well as the test of the new XTi SLR.

I am wondering if you could clarify some things. I am confused why the lenses that fit the aps sized sensor as found on the Canon Rebels will not also work on their cameras that utilze a full 35mm sized sensor? Is it perhaps because the lens must focus at a certain distance from the rear element and fill the sensor and that mission cannot be fulfilled on both sized sensors? Additionally, why do the camera makers carry the 35mm focal length of the lens on aps sensor cameras? This has consumers multiplying by 1.6 to determine the actual focal length that applies to the aps sensors and to describe the true focal length of the lens. Old photo buffs certainly understand what field of view a given focal length will provide with a 35mm camera, but given new photographers, using the 35mm "standards" would seem to confuse the process of selecting some lenses for their digital SLRs that use aps sized sensors. It is interesting that these lenses do not work with 35mm film or 35mm sized sensors, but carry the 35mm "standard" nomenclature with regards to focal length.

I hope that you can respond to this.

Thanks for all of your work and best regards,
Tom

PS: I think I have posted this in the wrong section, however maybe you can shift it over to the Canon section where it probably belongs. Don't think I can do that in an edit.

Hi Tom, welcome to the Cameralabs forums!

As you know, the APS-C-sized sensors used in the Canon Rebels, 20D and 30D are physically smaller than a full-frame sensor or a 35mm film frame.

Since one of the hardest things about making a lens is correcting it for good performance at the edge of the frame, it's clearly easier if the frame is that much smaller.

So Canon's EF-S lenses are only corrected for the smller APS-C frame, and wouldn't look great on a full-frame body. This correction also allows them to be smaller, lighter and cheaper than a full-frame equivalent.

Canon also exploited the fact the Rebels and the 20D and 30D have a smaller reflex mirror to allow EF-S lenses to extend a little further into the body - this again has optical advantages.

So the first reason is quality ahnd the second reason is physical incompatibility, which is why EF-S lenses can't be used on the older 10D, because even though it has an APS-C sensor, it has a mirror from a full-frame body.

To avoid physical problems, not to mention disppointment from quality, Canon gave EF-S lenses a slightly different fitting, so they physically can't be mounted on a full-frame body, or the 10D.

As for the question of focal lengths and labelling, that's one people have discussed for a long time. Most people understand what they get with a 35mm camera, hence the broad use of equivalent ranges. Very few people would say a compact has an 8-24mm lens for example - they'd say it had 35-105mm!

And where a lens is ONLY designed for a digital body, I agree, it makes sense to label it accordingly. So yes, since EF-S lenses will presumably ALWAYS be fitted to a body with a 1.6x crop, why not label them as such?

Probably the craziest example is Four Thirds, a system designed to be digital from day one with sensors using a 2x crop. So there was an ideal opportunity to label lenses with their equivalent focal length, but Olympus and the other Four Thirds guys decided not to.

It is a bit of a chicken and egg situation, but it doesn't look like we'll see a change in the near future!

What do other people think? Would you prefer lenses labelled with their true focal lengths or an effective focal length if they're only ever going to be fitted to digital bodies with a known field reduction value?

Gordon

Gordon,
I suppose all of the labeling of lenses is a bit academic in a sense.

WRT sensors: if a camera with an aps sized sensor (say 8 mpxl) can provide a good photograph that can easily be enlarged to say 8x10 or maybe even a little larger with good results...then moving to a camera with a 35mm sized sensor and more pixels really doesn't do much for the photographer - except give one bragging rights and allows one to spend more money on camera equipment. So the need for a camera with a more sensitve/larger sensor should be qualified by whether one can perceive much if any difference in the photo quality between the pretty common 8 to 10 mpxl smaller sized sensor and the larger "professional" sensors in more expensive cameras. Somewhat like comparing a 2 1/4 film vs 35mm film...if you don't enlarge the photos that much, most will not be able to see any difference. I think given my age and sight, I will end up being quite happy with my Canon XT for quite some time AND the quantity of new and used lenses and replacement cameras will be endless for me... Interesting stuff... I have a lot of old Nikon cameras and lenses to dispose of on ebay...who needs it!

Thanks for your comments before. I enjoy reading your tests.
Tom