Questions re: Focus

[linpelk]

This evening I took my two lenses out in the yard and took a couple of pictures of a pair of shoes. I placed my camera on a steady surface so there would be no camera shake (of course I had to physically push the button, but hopefully that wouldn't effect my experiment) and I used the same distance f/stop, ss, ISO. I used my 24-105 f/4L IS lens and my 50mm f/1.8 lens.

Why does the pair of shoes shot with the 24-105 lens appear to be further away when shot at 50mm even though the camera was in the exact same spot as when I used the 50mm lens?

I know I'm going to feel stupid when someone explains this, but I just assumed they would look exactly the same. Does it have something to do with the physical length of the lens?

I was actually trying to see the difference in how crisp the image was comparing the two lenses.

When looking at the picture, they both appear sharp, but when I use my loupe tool, I can see that my 50mm lens is slightly clearer. I was wondering though if that had something to do with the distance issue since it didn't seem like I was comparing the same image.

Here, I will post the two pictures. Maybe you guys can help me. Maybe there is a better way to test this. Any suggestions? Pardon my daughter's dirty shoes. They were sitting outside near me so I grabbed them simply for the sake of test shots.

These are SOOC, uploaded to flickr, then posted here:

This is the 50mm lens:

This is the 24-105 lens:

 

[Samanax]

Did you shoot the second image at 50mm on the 24-105mm lens?

Prime lenses are generally sharper than zoom lenses because they have less elements for the light to pass through.

 

[linpelk]

Did you shoot the second image at 50mm on the 24-105mm lens?

Prime lenses are generally sharper than zoom lenses because they have less elements for the light to pass through.

Yes, both shot at 50mm...I even checked my exif and it agrees.

I just figured since my zoom was L series glass (i.e. cost me a fortune at the time I bought it) that it should be at least comparable to a $90 lens! What's the fuss about L-series lenses if they don't take tack-sharp photos??

 

[musicaleCA]

You know, that's the reason people call the EF 50mm f/1.8 "fantastic plastic". (Personally, I have the EF 50mm f/1.4 USM, for the AF, better build, and slightly better optics.) As stated by Samanax, fixed focal length lenses will generally be sharper than zooms, just because there's less, well, lens. Less is more, after all.

I'm really scratching my head over the FoV difference. Everything in me is telling me that shouldn't be happening.

 

[Torus34]

It's not that a fixed focal length lens ['prime' lens] is somehow less lens that results in its ability to produce a sharper image. It is entirely possible to make a zoom lens with less elements than a fixed fl lens. It would not change the situation -- the problem inherent in designing a lens would remain.

The design of any lens requires that six sources of image degradation be considered and minimized. Among these are spherical aberration and chromatic aberration. Any lens design is a compromise -- an attempt to balance the six 'problems' and end up with the most accurate [distortion-free], sharpest image.

Solving this for a single fl is a difficult job. If you try to do this for a lens with a range of fl's, you'll find that you can get a 'best' solution for one point in the fl range but as you move away from that 'sweet' fl, there will be a fall-off in overall correction. The increase in the number of lens elements found in zoom lenses as compared to fixed fl lenses is due to the desire to minimize this fall-off.

 

[Dwig]

...Why does the pair of shoes shot with the 24-105 lens appear to be further away when shot at 50mm even though the camera was in the exact same spot as when I used the 50mm lens?...

They are different sizes for three reasons:

1. Image size is not controlled by the focal length, but by the distance between the expit pupil and the image plane.

2. The "50mm" makings on the lenses actually mean "somewhere roughly in the neighborhood of, but certainly not exactly 50mm".

3. The two lenses focus by different methods, the zoom by changing focal length.

4. There is not a single point in the lens from which the focal length is measured. There are separate points for measuring from the front and from the back. Only in very simple lenses do these two point coincide. The relative poisitions of these points will vary between your two lenses and this difference will have some impact on the size difference seen. It will be very small at these focusing distances, but in or near true macro ranges it can be a significant factor as it changes the effective distance from the subject to the lens. The physical size of the lens is not a factor itself, though the longer the lens from front to rear element the more likely there is a greater distance between these two points.

Your 50mm prime lens is probably very close to a 52mm focal length; most Canon and Nikkor 50's are actually designed that way. 50mm is just a rounded off number. The 50mm setting on you zoom is not a precise marking; its only accurate to about +-10% or about 5mm in this case.

The 50mm prime lens focus by moving the whole lens away from the film. This narrows the field of view. Your subject is reasonably close, close enough for the decrease in FOV (read: increase in subject size) resulting from the focusing to be significant in comparisons like this.

The zoom focuses by moving elements around. The effect is usually a reduction in focal length as you focus closer. This, combined with the effect of the shorter FL being "extended" further away from the sensor that it would need to be to focus at infinity results in an image that is either the same size or slightly smaller and the marked FL would imply.

If you redo the comparison using a subject at or very near infinity (at least 1000x the focal length (>150ft) you will eliminate the size differences resulting from the differing focusing systems. You will then be able to see the difference resulting from the accuracy/inaccuracy of the FL markings.

 

[linpelk]

Thanks for all your answers. I think it's pretty clear to me why the shoes appear to be further away in one pic compared to the other..

Now I'm just wondering why I spent $1000 on a zoom lens if it's not THAT sharp. If I would have taken these two pictures comparing an L-series "zoom lens" with a non L-series "zoom lens" would I really see a difference? Just curious about future purchases..

 

[dxqcanada]

A $1000.00 lens is not necessarily sharper than a $100.00 lens.

Fixed focal length lenses are cheaper to make in higher quality due to their lens design.
Zoom lenses have complicated lens designs and many more elements ... as they have to compensate for aberrations throughout the varying focal lengths.

That is why some of us stick to just using fixed focal length (prime) lenses.

I only have one zoom ... all the others (16mm, 50mm, 300mm, 400mm) are primes.

You probably will see a difference between an L and non-L.

 

[linpelk]

A $1000.00 lens is not necessarily sharper than a $100.00 lens.

Fixed focal length lenses are cheaper to make in higher quality due to their lens design.
Zoom lenses have complicated lens designs and many more elements ... as they have to compensate for aberrations throughout the varying focal lengths.

That is why some of us stick to just using fixed focal length (prime) lenses.

I only have one zoom ... all the others (16mm, 50mm, 300mm, 400mm) are primes.

You probably will see a difference between an L and non-L.

I guess comparing a zoom with a prime wouldn't give me a great comparison, but when shooting kids (which is what I mostly do) I feel like I HAVE to have a zoom. Just want to make sure that L vs. non-L zooms are going to make a difference for sharpness.

Thanks again for your replies!!