bit of theory (DOF and apperture)

B0rG

I can't believe I'm doing this instead of doing something else, but anyways.

Apparently some people here already know how to photoshop a fancy copyright, but somehow missed the basic stuff about apperture, depth of field and focal range. On any computer forum you'd usually get the RTFM answer, but there has to be RTFM to point to. There will be from today.

Attached there are 3 (2 x 3 + 4 = 10 all together) boring pictures of batteries sitting on a GO board within equal intervals. Pictures are taken at
1. 24mm (consider it your wide angle)
2. 99mm (almost 100 - tele)
3. 200mm - super tele.

White balance and exposure compensation are not corrected, as it's not the point. I didn't bother going higher than F11, as because of long exposures the shooting take all evening.

24mm - this by far is the most interesting one.
F2.8 - very shallow depth of field - 1st battery is completely out of focus.
F11 - all of the batteries are considerably in focus.

98mm - same story

200mm - almost the same.

DOF is the function of three main parameters: focal range, camera to subject distance and the apperture size. From the pictures we can see how apperture changes dof withing relatively the same camera to subject distance (0.2 - 2 meters). To summarise: increasing focal range will give you bigger bokeh effect than apperture. Changing apperture will increase the focus plane: closing the diaphragm - puts more batteries in focus, opening the diaphragm leaves less batteries in focus.

When focusing to infinity the situation will change - everything relatively far from the camera will be in focus. There are also DOF tables for every lenses - they can be found on a manufacturer web site.

The actual reason why I was doing these photos was to understand how focal range affects the relative sizes of objects on the picture. We can see that on 24mm there is noticable difference in sizes between the first and the last battery and on 200mm all batteries are almost the same. The picture gets flatten and you don't feel the distance between objects. Because of that nobody shoots portraits with focal length bigger than 135mm as you get very flat looking face. 135 is also the FL of popular prime lens.

sineadw

Thanks for that Don't suppose you could explain in plain english why a wider aperture or longer lens narrows the DOF? Not important - just being curious and nerdy.

B0rG

Optics, really.

A friend just done the post about it, the problem is it's on russian

This is the schema for open apperture


This is the schema for closed apperture


As you can see the object in focus is still in focus, but the light from the object out of focus is forced to be "thinner" (compared to wide open apperture) so it appears to be sharper.

the post is here
http://karanagai.livejournal.com/38877.html

Xios

B0rG wrote:

The actual reason why I was doing these photos was to understand how focal range affects the relative sizes of objects on the picture. We can see that on 24mm there is noticable difference in sizes between the first and the last battery and on 200mm all batteries are almost the same.

The differnce of size ain't the lenses, it's the angle you shot them at, there's no depth perception on the second and third lens.
But thanks for the info, potraits with 135 or lower.

B0rG

Xios,
not exactly true. Different focal length gives you different relative object sizes. There is a Bond picture thread with the picture of a gun very close to the lens and the guy holding it in the background. Gun barrel looks bigger (because it's closer). Same effect. And it shows better on wide angle lenses because of bigger object distance ratio.

You can do the same setup and see for yourself as I didn't bother properly aligning the vertial perspective. For me it was enough to understand the effect.

mikeanywhere

Nothing wrong with putting a nice copyright logo on a picture (said with tongue in cheek)

B0rG

Creativity aside, time to continue.

In first post we've seen that aperture affects the depth of field. Now it's time to show how focal length affects the dof. Attached are pictures of softdolls and cans of Bud (yes I do drink it from time to time) taken at various apertures and focal lengths.

Equpment used: 17-40 F4L, 70-200 F4L and 30D, ISO 400, Av and whatever shutter speed metering will give us.

200mm and 135mm are full images, the rest is cropped to match the field of view from 135mm shot. Shots at F22 were taken just for illustrative purposes, most of the time it's hard to imagine somebody using such apperture in the field. F4, F8, F11 are normal operational values of apperture. No sharpness applied - just white balance correction, crop and resize.

Dolls were around 3.5 meters from the camera (if we trust the focus finder) and bottles are approximately 5 meters from the camera. Mice and bear are at one plane and the gray cat is approximately 10 cantimeters behind them.

As we can see at 200mm at F4 the cat is thrown out of focus and you can barely distinguish the bottles. More or less reasonable picture comes out at F22, but as I said hard to see anyone using it in a field.

At 40mm however at F4 we get reasonably sharped background with a slight softness, but this is partly due to the lens being at the edge of the diapason, F5.6 is usually more recommended. At F8 - F11 everything is completely sharp. That's why they say that wide angles are sharp on all field of view. And that's the secret of digisoap being so sharp.

Happy snapping and post questions, as the only way to get me doing something is getting into an argument...

PS I really find some use for those 3 lamps I bought, found them good enuff, especially at E16 a pop in Woodies DIY. They are 160W, and give a slight yellow tint, which is easily white balanced. Great stuff for small macro studio if only I could manage the to put them on some sort of tripod. They off course made in the People's Republic of China, so the overall quality leave a lot to be desired of, but at 16 quid each, I think they are great.

sineadw

"this is partly due to the lens being at the edge of the diapason"

Ok I was avidly following you until then... diapason? You've lost me. Sorry..

B0rG

sineadw wrote:

"this is partly due to the lens being at the edge of the diapason"
Ok I was avidly following you until then... diapason? You've lost me. Sorry..

Don't let that stop you

Hmm, I checked for that word in dictionary, dictionary said it's translated one to one. Anyways - it's just another word for range. In that sentence I'd use diapason in russian, so I just translated it.

kuroino

I am that friend who wrote about apperture affecting the DOF in Russian.

I see you discuss how the focal length could affect DOF.

The fact that DOF decreases whith increasing focal length could not be so easily illustrated as in the case of the apperture. It comes from the basic equation of 1/f = 1/d + 1/r, where f is a focal length, d is a distance to the object in focus and r is a distance between the focal plane of the lense and the film plane. It involves a little bit of math to see. And it seems to break on values of d close to f.

digitalage

Borg in your first post I know what your trying to do but your tests are flawed as the 24mm shots are taken at a different angle to the 98mm and 200mm, and for the record a 200mm would'nt be considered a super telephoto! anything over 400m would. I'm not sure why you bothered with this as there are plenty of good websites that can explain it properly maybe you drank to many bears in the bear photo

kuroino

> 24mm shots are taken at a different angle to the 98mm and 200mm,

I think the problem is that his camera was not looking straight, but somehow down, so he had to lift the lense higher when he was moving the tripod away for longer focus distances.

24mm is just too different to 98mm in this respect.

B0rG

digitalage wrote:

Borg in your first post I know what your trying to do but your tests are flawed as the 24mm shots are taken at a different angle to the 98mm and 200mm, and for the record a 200mm would'nt be considered a super telephoto! anything over 400m would. I'm not sure why you bothered with this as there are plenty of good websites that can explain it properly maybe you drank to many bears in the bear photo

Fair enough.

The only reason why I've done it is by trying it myself made my hands remember the effect, and I can feel it without thinking about it. Plus I gave a try my new macro lamps and tried a thing or two in comosing the studio shots. Some people benefited from it and thanked me, obviously it wasn't you.

Can you elaborate how flawed they are? Preferably in numbers (this is school stuff really)... Angle was due to the size of 24-70 - I had to lift it from the object. Thing is this effect depends on relative (to focus length) distances from object to object and object to focal plane of the lense. Hence I needed to shoot close. The "perspective angle" has some effect but no more than 5%, again you can calculate it yourself (school trigonometry again) if you really wanna argue. I'd say even with the angle it is still acceptable to demonstrate the effect.

Effect can also be seen by human eye - in fact you see it eveyday: close one eye (to flatten picture a little bit), stand close to the line of buildings of the same height, and you will notice that the further building looks smaller. Common sense really.

kuroino

The best way to demonstrate this effect is just shoot a small object (a cup) standing in front of a larger one (a picture). The batteries were bad choice anyway.

Actually I think you are rarely interested in what happens to objects in front of the object you are focusing on. Only a proportion of the main object to the background is of practical interest.

digitalage

Borg theirs enough written about this boring stuff on the net, its a photography forum not maths, once your happy with your lenses and how they operate, now show me your pics

B0rG

I'm always open for advice on point of view, ligthing and composition for this type of shot.

http://picasaweb.google.com/peterperov/StPetersburg/photo#4981425221840535570

Leave aside the cluttered river - photoshopping it out will not help the picture at this sage. I wanted the theme to be "picture in picture" but details and the theme is still unclear to me. At this stage I'm just snapping away two objects, untill I'm lucky enough to catch the balance between the painting and the what's being drawn - in this case relying heavily on the painter's choice of the point of view. Or may be the theme is "sneaking"? I know I'm trying to jump higher that I actually can, but this is what's making it interesting to me.

Here's Cartier-Bresson solution (may require magnum membership but it's free)
http://www.magnumphotos.com/CorexDoc/MAG/Media/TR3/F/W/C/L/PAR96724.jpg

Subject is similar but not the same. His theme is "painter" not the one I've chosen.

The interesting thing is, if you know St-Petersburg, the painer is facing one of the most beautiful parts of SPB architecture - the painter is seeing the following buildings:
Admiralty
http://picasaweb.google.com/peterperov/StPetersburg/photo#4972509066167451666
Isaacs Cathedral
http://picasaweb.google.com/peterperov/StPetersburg/photo#4972523001004818450
http://picasaweb.google.com/peterperov/StPetersburg/photo#4976212136759001106

As there are are magnificient buildings in the area I'm sure they caught Cartier-Bresson's eye. And that makes me wonder why he's chosen that point. Did he assume the viewer knowledge of what's being drawn or was he just photographing the painter?

digitalage

Borg I think your picture would have been better IMHO if you used a lower F-stop, focus on the artist and blurred out the background as there is too much going on, I would also lighten up the foregraound a little and I think B&W would work well with this pic.

B0rG

one of suggestions was to crop the artist out completely, leaving only the painting. I don't wanna loose the color, as oil paintings on the street do not really work in black in white. Cluttered background yes, but as I said leave it aside for a moment. After all god gave us photoshop for a reason. And as the composition is complete s h i t e clearing up background won't really help the picture anyway.

I've done similar shot where painter is painting the statue in the park - a lot easier, but the shot didn't work completely. Anyways it's been 1.5 years since I took that shot, prolly in 5 years I finally make what I like.

And may I remind you that this is theory thread... May be I should suggest to moderators to make a theory subsection in the forum, same as they did with digital darkroom...

Dimy

You have a very theoretic and mathematical approach to photography Borg. I learned most about photography by experimenting, taking the same shot many times but with different settings, different composition etc until I was satisfied.
Now that I actually know how my camera works and what effects certain settings create my approach is much more natural, I don't really need to think anymore how to get the picture I want.
Sure Photoshop is a gift from the "gods", but if I want to have a blurred background I make sure I take the picture with a wide apperture and not photoshop it in later.
Everybody has his own learning curve and methods though, so if this approach works for you who am I to slag it off . It's an interesting read nevertheless, but kinda boring as someone already mentioned. Good luck with your experiments and I'm sure we'll see the results here.

B0rG

Dimy,

I will not fall for another pointless argument about "academic vs creative approach to learning photography" no matter how hard you try By now I am pretty much aware where everybody stands, so not much point to continue, because... well... I'm not getting anything out of it. I'd rather spend time going through Cartier-Bresson or Ansel Adams pictures, reading the book about composition or playing with different lighting and cropping effects to see how they affect perception of the image.

Or I can go to one of the russian photography sites and check out top 100 photographs of the week chosen by people with good taste and education, who among other things can explain why they prefer one picture over another.

Or there are a bunch of tutorials on horisontal and vertical perspective that I wanted to make. Life's short and there is plenty of stuff to do, you see?

But since friday is a good time for having good argument how about Canon vs Nikon? Or Windows vs Linux, or may be Buweiser vs Guinnes topics? I like the firm and crispy taste of american brewed beer. In fact this is probably what I will enjoy after work. Though I do enjoy probably the best lager too...

Dimy

You missed the point completely I wasn't trying to start a new arguement, instead I said if your approach works for you who are we to argue about it :rolleyes:
I was just trying to point out that different people have different approaches to photography, others have a very technical approach like you have while others have a more natural approach... and then there are others with a little bit of both.

kuroino

I guess b0rg's method is to learn while teaching others. Very good method. I don't think theory harms anyone as long as you don't mistake it for the photography itself

FreeAnd..

I don't think theory harms anyone as long as you don't mistake it for the photography itself

Excellent point, as both are two separate things completely..I think it was you B0rg who may have used the analogy of Critcs and Artists in a different context...Fits this to a tee, you shouldnt mistake artistic knowledge for artistic talent...one can be learned as for the other, well you can work that out for yourself.

Personally, I will leave the qualified critics spend time over the technical in's and outs, I am not in the Art Critic profession but I can judge what works personally for me and also why...