And another thing "longer reach"

[Adrian]

I keep reading on the internet, including here, that a a 1.6 crop factor Canon 7D (say) has a "longer reach" than a full frame 6D or 5D Mk III. Hence a 300mm tele becomes 480. I was even told this in a respected cameral shop.

Surely this is nonsense? It seems mathematically illogical to me.

The whole thing about a crop sensor is surely that it is a cropped part of a full frame (the central bit). Hence, if I took exactly the same shot on a full frame using the same lens, and then cropped the shot on my computer, it would give exactly the same "zooming effect". (Leaving aside IQ, noise etc issues etc between different sized sensors).

It seems to me that if I am right, there is no real advantage in having a crop body for so called "longer reach". You might as well take the same shot with a full frame and then crop it in processing.

I do realise that crop cameras are often used with smaller and lighter glass, so there are carry around factors to consider, as well as cost. But I am looking purely at the so called advantage of the crop sensor.

Have I misunderstood?

It seems to my that if I have a FF, then whatever the widest end of my lens is, then the camera will deliver that. But that same lens on a crop camera, will not us the full capacity of the lens: hence clear disadvantage. The converse is surely not true at the other end of the lens as the full frame will still be able to resolve the same image as the crop, with processing on the computer?

[Inkanyezi]

No you have not misunderstood, and this mumble jumble of the focal length virtually changing causes much confusion, particularly among people who don't understand the purely physical background about what a focal length is.

However, as you hint, there are other factors also involved.

A lens made for the 24x36 mm format would not need as much resolving power in the centre as a lens made for a substantially smaller image circle. Hence correction is often such that the lens delivers reasonable sharpness for the large format, while a lens that is designed for the smaller format may have higher resolution over the small area it must cover. In essence, if you use a "FF" lens on a "crop" camera, it might not be as sharp as a lens designed for the smaller sensor size. The lens that is designed for the smaller size can be better corrected for some optical errors, because it does not have to cover a large image circle.

So even though a 50 mm lens for the "full format" will cover the same angle on a 4/3 camera as a 50 mm lens designed for the much smaller format, you could expect better performance on the small format from the lens that's designed for it. Hence using a lens designed for the 24x36 format will not be optimal on a 4/3 camera.

The other considerations are fully valid. A smaller sensor can use smaller lenses (as they need not cover a large area), and the whole outfit is smaller and weighs less. But of course there are advantages of the larger size. The larger size can deliver better image quality, provided most other factors are comparable. The larger size also with its longer lenses will provide more bokeh, and less depth of field, which is often desired. The larger sensor is more sensitive to light, hence you can crank up ISO more before you experience excessive noise.

But your initial ramblings hold water. The focal length of a lens is totally independent of what chip you put behind it.

[DanK]

Adrian,

It's just trigonometry: a crop sensor gives a smaller field of view (FOV) and therefore crops away the edges of the image. You are therefore correct that if you cropped the image from a FF camera, you would get the same perspective and reach (although, in most cases, many fewer pixels on the subject). The "reach" discussion assumes that you want to fill the viewfinder or sensor with the image, and if you are going to do that, yes, you get more reach with a crop sensor camera, since you can stand further away and still fill the smaller area. This is just a function of the smaller FOV of the lens on a crop sensor camera.

The issue of pixel density is a big one in my mind. Yes, you can crop a FF image to get the same reach that you would get with a crop-sensor camera, but in many cases, that will leave you with a LOT fewer pixels on the image.

Re Urban's comment:

while a lens that is designed for the smaller format may have higher resolution over the small area it must cover

That may well be true be in some cases. You will find the same issue discussed in this post, which is one of the best discussions of the tradeoffs I have stumbled upon. However, in practice, this has not turned out to be an issue--as far as I could tell--in the case of any of the lenses I seriously considered for my crop sensor camera. That is, looking at reviews, I did not find cases where the EF-S (crop only) competitor had substantially higher resolution in the center. In fact, it is often the reverse, if only because manufacturers often produce their most expensive lenses for FF format. For example, I have both the 100mm L macro and the 70-200 f/4 IS, and both function superbly on a crop. I have one A/B comparison: I shoot macros with both the EF-S 60mm and the EF 100mm L. If I recall, the latter has better resolution results for the center, but the fact is that both perform superbly, and you would not be able to tell the difference if I gave you a bunch of prints to compare.

Dan

[rpcrowe]

Adrian...

IMO, Dan has hit the nail on the head when he remarked (and I am paraphrasing) that top-line lenses designed both for crop cameras and for full frame cameras can perform "superbly" on both full frame and crop cameras.

I use both the 17-55mm f/2.8 IS lens (EFs - designed for crop cameras) and the 70-200mm f/4L IS (EF - designed for full frame use) on my crop format cameras (40D and 7D). They both perform exceptionally well as do my 300mm f/4L IS and 400mm f/5.6L lenses.

Frankly, I don't think that (considering image quality as the only criteria) that you could tell the difference between the images from my 17-55mm f/2.8 IS lens from those shot with my 70-200mm f/4L IS.

Additionally when many people consider the image quality differences between top-line lenses, they are splitting hairs regarding those differences in image quality. I don't think that the differences would be evident in real-life, day to day shooting.

I would recommend that a photographer get a top-line lens for whetever camera that he or she uses and forget about differences in image quality; simply work to get the best results out of that camera/lens combination. I think that I am a pretty decent photographer but, I am sure that I do not get the optimally best shots which my camera/lens combinations such as my 7D and 70-200mm lens are capable.

BTW: I was also getting some pretty darn good imagery from my 24-70mm f/2.8L lens while I was shooting with that lens on my 1.6x cameras. That's another example of a EF lens meshing fine with a 1.6x system.

[Adrian]

Thanks. And for the link. I am still wondering about this issue because, as per my other thread, I plan to be doing quite a bit of night shooting in March (plus some practising before then). This is of the night sky and will be long exposures on a trip, where I will want to capture a wide field of view and focus (manual I expect due to very low light) at infinity.