Quick question about full frame + lenses

[Ksanti]

I'm not genius on optics, so forgive me if this is really stupid, but I've been thinking about this:

Whenever I've seen people considering 5DMK2's, D700's etc. on various forums, a word of advice has pretty much always been to get lenses that "do the body justice", and while obviously this may be true, surely a full frame body is less punishing of a soft lens?

If you're displaying at the same size/resolution, then the lens is focusing over a larger area for each relative area in the final image. Surely that means that to get an image of the same sharpness (ignoring distortion, vignetting etc.) on full frame, compared to crop, you don't need as accurate/tight a focus from the lens?

Given the lens is the same distance from the sensor regardless of sensor type (I assume, or at least similar), then a lens that is soft by, say, 2mm (i.e. no area smaller than 2mm is focused on the sensor), will show up more on a crop sensor which has a smaller overall sensor size?

I /think/ I'm right, and of course I'm not saying full frame is a substitute for good glass, but in this one aspect, would a full frame give a sharper (with wider FoV) image from the same glass?

Just wanted to check as I'd only ever heard the "make sure the glass does it justice" comments on 5d/1d/d700/d3 threads, though of course that could easily just be a result of these bodies being such huge investments compared to most cropped cameras?

 

[StewartR]

I don't think this is stupid at all.

I don't want to talk about comparing sharpness across different sensors, because it's all too easy to get bogged down in issues such as depth of field, circles of confusion, and diffraction - all of which affect image sharpness but to differing degrees depending on the sensor.

However I think it is fair game to talk about resolving power, and it is immediately obvious that a high-MP crop sensor can resolve more detail than a low-MP full frame sensor. For example a Nikon D3 or D700 has about 1200 pixels per cm, and a Canon 5D Mk II has about 1500. But my old Canon 40D has about 1700, and a Canon 7D has about 2300. There are surely situations where a 7D is getting more resolution from the lens than the D3, and in this sense the full-frame sensors are less demanding. As you suggested.

But having said that, there are aspects of lens performance where full frame is more demanding. Edge and corner sharpness are much more important on FF than on a crop, and FF is much more vulnerable to vignetting / light fall-off. And of course, it takes a quality lens to deliver on these two fronts.

 

[LongLensPhotography]

As said above, corner performance on FF is an important aspect of quality. Crop sensors will hide these imperfections, so technically a lens that is only good / sharp in the centre will be fine on APS-C. One such example might be Canon 24-85mm USM.

However centre sharpness is as important as ever, and it is very unlikely that cheap glass will resolve well and have good corners on APS-C, if it has hard time on full frame, or vice versa. Even some L glass can just about resolve on high density sensors wide open.

So I am coming to a conclusion that all recent dSLRs require high quality glass. Once again, pro glass means simply 'fast' and consumer - 'slow and variable' but sharp. Canon 15-85mm (slow but sharp), and the end of non-L full frame line mark this turning point.

 

[arad85]

Full frame isn't about the lines per inch of resolving power (certainly all current Canon have more pixels per area on crop than full frame) but it is about delivering that performance over a significantly larger area. A full frame piece of glass (i.e. EF in Canon speak) that is "OK" over the full frame sensor may well be great over an APS-C sized one. Put it on a full frame sensor and it may well illuminate it but also show its limitations.

On the FF glass argument, it is all about resolving power across the full frame projection area - not about resolving power on APS-C sized sensors.