Crop sensor question - edge sharpness

This isn't clear to me.

Let's say you have a lens that is sharp in the middle but the edges suck.

Does this improve if you mount it on a crop sensor body?

Not necessarily because to get the same depth of field on a 1.6x crop camera, you would open the aperture by 1+1/3 stops or to say it the other way, when on full frame, you would stop down 1 and 1/3 stops further to get the same result with a 1.6x longer focal length when taking the picture from the same distance.

So, for example, a 70-300mm f/4-5.6 zoom that would be used at 188mm f/5.6 on the crop camera would be used on the full frame camera at 300mm f/9 to make the same image. Stopping down to f/9 might improve the edges on the full frame image to match the crop camera at f/5.6. To compensate the shutter speed loss you would raise the ISO on the full frame camera also by 1 1/3 stops. You would end up with a very similar image but with the full frame one looking a bit sharper due to less magnification for the same print size.

uOpt wrote in post #11961791
This isn't clear to me.

Let's say you have a lens that is sharp in the middle but the edges suck.

Does this improve if you mount it on a crop sensor body?

That's quite a vague question really. If you put it on a crop sensor and don't change anything else, you will get just the central portion of the image you had with a FF camera. Often, on lenses where the quality falls off as you move away from the optical axis, the central part of the image is "better". - you just don't get edges (i.e. the sucky bits) so the part you do get can be better. If that's the image you wanted, that's fine. So it basically depends on how much the edges suck - they get better if you stop down. There are too many variables for a simple answer beyond this kind of thing. Or do, you have a more specific question ?

No, I was just thinking too complicated after making diagrams on paper.

Very obviously the answer is yes. You just cut out the edges that aren't sharp. It doesn't matter how you cut them out and making a diagrams with light rays isn't required.

Does a lens with lousy edge sharpness really get better as you narrow down aperture? I am talking about edge sharpness problems for objects that are in focus (have the same distance as the thing you focus on). I don't think so. If the glass at the edges just isn't shaped precisely enough you can't focus that up, no?

uOpt wrote in post #11961935
Does a lens with lousy edge sharpness really get better as you narrow down aperture? I am talking about edge sharpness problems for objects that are in focus (have the same distance as the thing you focus on). I don't think so. If the glass at the edges just isn't shaped precisely enough you can't focus that up, no?

That isn't how optical imaging works. All of the glass is used for all parts of the image. There is a light ray from each point on the subject, through each part of the lens which is then focused to a point on the sensor. Items at the edge of your picture don't just use the edge of the glass. As you close down the aperture, you are reducing the rays that are striking the lens elements at different angles so that all of the rays pass through the smaller and smaller iris and excludes the other rays that are at different angles. This does sharpen the edges of the image quite a bit.

Have a look at any of Canon's MTF charts for the lenses to see this effect. There are curves for wide open and for f/8. Notice how much higher the f/8 is, especially near the frame edge which is indicated by the larger radius on the horizontal axis.

Oh I knew that part. But what happens if the individual glass elements are made cheaply and only the center is actually at the angles you want and the outer parts of the glass are "off"? I think that must be the reason why some lenses have better edge sharpness than others.

macroimage wrote in post #11961813
Not necessarily because to get the same depth of field on a 1.6x crop camera, you would open the aperture by 1+1/3 stops or to say it the other way, when on full frame, you would stop down 1 and 1/3 stops further to get the same result with a 1.6x longer focal length when taking the picture from the same distance.

So, for example, a 70-300mm f/4-5.6 zoom that would be used at 188mm f/5.6 on the crop camera would be used on the full frame camera at 300mm f/9 to make the same image.

Why would I take the aperture from 5.6 to 9.0 on the FF? f/5.6 should give me the same light/exposure time at 188mm and 300mm. The f-stop already compensates for the different focal length.

I don't see why the FF versus crop matter for bokeh. The light rays that hit the crop area come in all over the lens in both cases.

You would go from f/5.6 to f/9 to give the same aperture diameter at 188 and 300mm to get the same depth of field at the same distance. It isn't to get the same exposure brightness, it is to get the same composition. To get the same shutter speed, you would need to go from ISO 100 to ISO250 on full frame to compensate for the reduced brightness at f/9 compared to f/5.6.

It would be a bit silly to compare 188mm f/5.6 on a crop camera to 300mm f/5.6 on a full frame since the latter would give much shallower depth of field and look different.

By stopping down you would sharpen up the corners some so switching to a crop camera might, but not necessarily remove the problem areas since you must do different things to get the same image.

The point being that when you change camera formats, you must do different things to get the desired image so comparisons aren't as simple as a simple cropping of the frame.

macroimage wrote in post #11962267
You would go from f/5.6 to f/9 to give the same aperture diameter at 188 and 300mm to get the same depth of field at the same distance. It isn't to get the same exposure brightness, it is to get the same composition. To get the same shutter speed, you would need to go from ISO 100 to ISO250 on full frame to compensate for the reduced brightness at f/9 compared to f/5.6.

It would be a bit silly to compare 188mm f/5.6 on a crop camera to 300mm f/5.6 on a full frame since the latter would give much shallower depth of field and look different.

Are you sure you need to keep the absolute aperture diameter the same to get the same depth of field between 188/crop and 300/FF?

I thought you would keep the same f-stop to have the same DOF.

That means the longer lens gets the bigger diameter but due to the narrower vision the same bokeh results.

Hmmm....

Consider the case of just cropping the final image from the full frame camera. In doing so, you have reduced the depth of field when viewing at the same size since you had to magnify the image 1.6x more to reach the same print or viewing size. The blur is magnified further.

Remember there is an f squared term in the depth of field calculations.

So if you take a full frame image at 188mm and then cropped the image by 1.6x, you would have a similar image to taking the whole frame at 300mm but you magnified the 188mm image 1.6x more to reach the same viewing size after cropping. This partially compensates the depth of field increase from using a shorter focal length, but not completely. You would still have to stop the 300mm down further to match. It is fairly easy to try it for yourself.

Or have a look at a more extreme example of a Canon S70 vs a Canon 1DmkII. In this case after matching the field of view, the 1DmkII had to be stopped down to f/13 to match the S70's f/3.5.
http://www.clarkvision​.com/articles/dof_myth​/index.html

Hmm, that makes more sense.

And yes, I can actually try this myself with just my 40D. I can simulate a "crop-crop" camera by cutting out a section in the middle and then see what f-stop I need at what length to match the bokeh.

Damn. I was looking for an excuse to get a 5D just for the testing

Thanks so much.

Yeah it's pretty easy to see when testing. 25mm shot at f/4 and then cutting out an area that would be 40mm focal length gives very roughly the same bokeh as shooting at 40mm with f/7.1

It's not a perfect test since edge sharpness of the lens kicks in but there it is.

Thanks again.

Sounds like you mean depth of field or amount of out of focus areas in the image.

Bokeh is something different - it refers to the quality, not amount, of the blur.

Right. Sorry.

In any case this did it, now I want a FF camera

uOpt wrote in post #11967525
In any case this did it, now I want a FF camera

Sorry about that.
"where ignorance is bliss, 'tis folly to be wise," - Thomas Grey 1742