Fast Lens, Depth of Field

JSPhotography

I have an indoor horse jumping event coming up. I shot another for practice last week. I feel like I need a faster lens so I was thinking of renting the Canon 70- 200 F2.8. In looking at my pics though, I am allready seeing some DOF issues with a horse and rider coming straight at me for example. My question is - If I want to shoot at say F8, at a given amount of light will a 2.8 lens alow me to use a faster shutter than a 5.6 lens?

raptorcaptor

At F8 the shutter speed will be the same on both lenses.

Enitsugua

JSPhotography wrote:

I have an indoor horse jumping event coming up. I shot another for practice last week. I feel like I need a faster lens so I was thinking of renting the Canon 70- 200 F2.8. In looking at my pics though, I am allready seeing some DOF issues with a horse and rider coming straight at me for example. My question is - If I want to shoot at say F8, at a given amount of light will a 2.8 lens alow me to use a faster shutter than a 5.6 lens?

Nope. f8 is f8. If you want more light from the 2.8 lens than what you get from the 5.6 lens, then you have to shoot the 2.8 lens at a larger opening ("smaller" number--remember, these are denominators of the fraction 1/n) than 5.6. Larger openings = more light but also equal less DOF.

Higher ISO will allow you to shoot at a smaller (bigger number) f stop, regardless of lens. Better results at a higher ISO usually entails a better camera.

The 2.8 lens will be brighter to look through since you are looking through the lens at its maximum aperture (it stops down to the shooting aperture when you press the shutter to take the photo). It will also likely produce better quality of images at 5.6 than the 5.6 lens because the 5.6 lens is all the way open at 5.6, while the 2.8 lens is stopped down 2 stops at that point. Most lenses produce better images a stop or two from wide open. Finally, the 2.8 lens is a pro lens and is likely to be able to focus a little more quickly than the 5.6 lens.

But, no, the 2.8 lens does not give more light at f8 than the 5.6 lens.

JSPhotography

Well, that is what I thought but not what I was hoping for. I still think it may be the way to go. So I will just use the 200mm to shoot farther away so I can increase the DOF as much as I can while using the F2.8 to get the fastest shutter possible for the available light. My other option is my 50mm, 1.8 but that is only good for the up close stuff and leaves me with the DOF problem. The rest of me lenses are all in the 4 to 5.6 range. Any issues with this plan?

g_smith

JSPhotography wrote:

Well, that is what I thought but not what I was hoping for. I still think it may be the way to go. So I will just use the 200mm to shoot farther away so I can increase the DOF as much as I can while using the F2.8 to get the fastest shutter possible for the available light. My other option is my 50mm, 1.8 but that is only good for the up close stuff and leaves me with the DOF problem. The rest of me lenses are all in the 4 to 5.6 range. Any issues with this plan?

It may also be worth noting that regarless of the lens the DOF will be the same at any given object size and f/stop. For example, if you photography two horses with your 50mm and 200mm at say f/3.5 and they just fill the frame then the range of apparent image sharpness (circle of confusion as it is often called) will be the same. However, the wider angle will have more background in the photo while the 200mm will isolate the subject much better.

A 200mm sounds like a good choice but very likely stoping down to maybe f/3.5 will significantly improved image quality. Focus on the eyes of whatever is the center of attention in the photo and DOF may not matter much. Increase ISO to 800+ if you have a camera that controls noise well to increase shutter speed. I use Nikon's 70-300mm f/3.5-5.6 zoom and a D700 for sporting events with great results and would not discount your slower lenses.

Good luck

kofa

g_smith wrote:

It may also be worth noting that regarless of the lens the DOF will be the same at any given object size and f/stop. For example, if you photography two horses with your 50mm and 200mm at say f/3.5 and they just fill the frame then the range of apparent image sharpness (circle of confusion as it is often called) will be the same.

Are you sure about this? I haven't done the maths, but these shots seem to contradict what you say - either that, or I've misunderstood you.
http://www.wlcastleman.com/equip/reviews/d80/

Check out how much blurred the background lights are with the 200 mm lens at f/2.8 than with the 50 mm lens, also at f/2.8 (the photos also nicely demonstrate the change of perspective that you've described). Left: 200 mm, right: 50 mm (check the URL above for high-res images). According to the site, 'All images are full frame', so cropping was not used to modify apparent size.


Kofa

pathfinder

Here is Michael Reichman's demonstration that depth of field is not a function of focal length, despite what lots of folks think ( incorrectly )

Here is even more by Michae

This has been discussed in Pop Photo two or three times over the last 25 years, as well as on the web. Michael's links are just the easiest for me to find quickly, as I have them in my bookmarks. One way to think abut it, is to realize that a given focal length can be a normal lens, a very wide angle lens, or a very telephoto lens, depending on what your sensor ( film ) size is....... A 50mm lens is a telephoto for a point and shoot, a normal lens for a 35mm camera, and a Very wide angle for a large format view camera.

kofa

Dear pathfinder,

I haven't had time to read the articles - thanks for posting them, I will read them as soon as I have the time.

However, if I look at these photos:

and ask myself:
- is the tower equally blurred on both?
- are the branches of the trees equally blurred on both?

I'd certainly say 'No'.

On the other hand, looking at the 17 mm crop, where the tower is about the same size as on the 100 mm shot, the amount of blur is rather similar, too. I still have to think about the implications of that, though.



Now, before I post any more, I'll go and do my homework (reading the articles and doing the maths).

Thanks for the info and food for thought.

Kofa

craig_d

pathfinder wrote:

Here is Michael Reichman's demonstration that depth of field is not a function of focal length, despite what lots of folks think ( incorrectly )

This keeps coming up no matter how many times people who actually understand the math shoot it down.

What Reichmann is really demonstrating here is a special case of DOF -- the fact that a change of focal length, balanced by a corresponding change of camera-to-subject distance (i.e. focus distance) to keep the subject the same size, will produce roughly equivalent DOF. An increase of focal length reduces DOF, but an increase of focus distance increases DOF. (If the latter were not true, there would be no such thing as a hyperfocal distance.)

It is true that DOF is not a function ONLY of focal length, but who ever claimed it was? Everyone who understands how DOF works knows that it is affected by focal length, aperture, and focus distance. Reichmann is not in any way disproving this, or even trying to. To keep his little gremlin doll the same size in shots taken at a variety of focal lengths, he moves the camera. So focal length and focus distance are both changing. That this results in equivalent DOF is useful to know but it does not in any way contradict the conventional understanding of how DOF works.

pathfinder

Once again, as I said "One way to think abut it, is to realize that a given focal length can be a normal lens, a very wide angle lens, or a very telephoto lens, depending on what your sensor ( film ) size is....... A 50mm lens is a telephoto for a point and shoot, a normal lens for a 35mm camera, and a Very wide angle for a large format view camera."


Here is another link saying exactly what you and I are saying - read down to the paragraph titled DOF versus focal length. Once again, if the subject is rendered the same size on the film plane, the DOF is not a function of focal lengths. If we require the subject to film plane remain constant, and make the subject much smaller as the focal length gets shorter, then DOF WILL be greater with shorter focal length lenses. This is precisely WHY we use shorter focal length lenses, so called wide angle lenses - to make the subjects smaller so they will fit within the confines of the viewfinder, film format, film plane.

Here is another excellent link confirming what I said Read down to Clarification: Focal Length and Depth of Field and you will find a table of calculated DOFs for various focal length and draw your own conclusions.

Knowing this information is useful. Here is one way. If I am shooting skiers flying down the hill towards me at a high rate of speed, is it better to shoot with a 500mm or a 200mm lens in terms of depth of field? Maybe I can't get close enough safely with 100mm. IF THE SUBJECT IS THE SAME SIZE IN THE VIEWFINDER with each of those lenses, my depth of field will be the same. The angle of view will not be the same of course, as 500 is a stronger telephoto for a given format, than a 100 or 200mm lens.


I am sure I can find more links supporting my post, but I think my post is confirmed adequately.

Thunder Rabbit

Howdy.

Here is a link to a Depth of Field Calculator.

Fiddle around with it and things should become a little clearer.

pathfinder

TR, your link is to this thread

adbsgicom

Proably this link:
http://www.dofmaster.com/doftable.html

pathfinder