DOF, f Stop and reading the lens

[New Hampshire]

Ok, I am sorry for having to rehash the usual DOF, f Stop stuff. I have been reading all these tutorials online for DOF and f stop and feel I am getting a pretty good grasp on it. However, one site I was reading started talking about using the DOF guage on you lens. The lens I am asking this question on is a Sigma 35-80mm, and it goes from f/4 to f/22 (or as the front of the lens calls it 1:4-5.6.) First off here is a picture of the barrel of the lens:

Ok, the site I was reading made perfect sense to me by saying that you first focus on your object. Then, by looking at the DOF guage the f/stop you are using will corrospond to to the distances of what will be in focus....i.e. as it is set in this picture above, using f/22 will put from 2.5 feet to 15 feet in focus. That I got. The problem, however, is that this lens' guage stops at f/11 and next goes into the "R" followed by "R" with the line above it, the thickwhite line (which is the indicator mark, that I know) back over to f/22 before moving back in the opposite direction. I guess the question I really need answered is how do I read all this confusing stuff inbetween the two f/11 marks?

Also, this question is about using infinity. I read this from the "luminous-Landscape" site:

"If you focus so that the infinity symbol is aligned with the engraved DOF mark for the aperture you are using (the right hand one of the pair), you will be at the hyperfocal distance for that aperture. Also, the left hand aperture marker for the aperture that you are using shows the closest point that will be in focus."​

The problem with this is that the infinity symbol goes from the LEFT side f/22 over to the white indicator line. What this means is that if I set the infinity on the white indicator there will be no f/stop values on the left side of the DOF guage, only the right side. Is that where the extra f/22 comes from just to the right of the indicator mark?​

Man, I hope this question makes sense. I am really getting confused trying to read the lens by applying what I have been reading on f stop and DOF.​

Can ANYONE explain this to me? Its probably a simple answer that is lost on me or way over my head. Hopefully you can help me make sense of it :blushing:​

Brian​

 

[fmw]

Sure, the problem is that the gauge becomes anywhere from hard to impossible to read and inaccurate as the aperture opens. The lines get closer together and that compresses everything. At the larger apertures, you need to use the DOF preview function or study an image on the LCD screen if it is a digital.

 

[markc]

When using infinity and hyperfocal distance, you use the leftmost f#, not the center line. If using f22, you would put infinity over the left 22. You would then have acceptable focus from ~3' to infinity.

Is that a Nikon lens? Do they focus the opposite way from Canon? On Canon lenses, the IR focus mark is just to the right of the main focus mark. On the Nikon lenses it may be just to the left. The R with the line may be the focus mark for focusing when taking infrared shots. I don't know what that extra "R" or "22" is for.

 

[Steph]

I am not quite sure I understand all of your question but I will try to answer anyway. The DOF scale on a lens depends on the focal length. As you have a zoom lens, they tried to put 2 scales in one. The outer pair of f/22, f/16, f/11 markings correspond to the widest setting on the lens (35 mm) whereas the f/22 marking to the right of the indicator correpond to a longer focal length (50 or 80mm, you need to find out on the lens manual). There also should be a symetrical f/22 marking to the length of the indicator but they did not have enough space to write it. As for the Rs they are probaly focussing indicators when you use IR films (check your manual).

Finally, to try to make it clear, if you take a picture with your lens at 35mm, at f/22 whith the focussing ring as set as it is one your picture, evrything should be reasonably sharp between roughly 0.8 and 5 metres.

I hope I did not make it even more confusing...

 

[markc]

Oh right. That makes sense. It's been such a long time since I've used a zoom that I forgot about that.

 

[New Hampshire]

Ahhhh, ok you guys have started to clear things up quite a bit! MarKC, it is a Sigma lens for the Pentax K mount ( Im pretty sure it is an auto focus/auto exposure model but I am using it on a strictly manual body.) Steph, there is one problem with checking the manual.....I don't have it. Unfortunately this lens was bought (GASP!) almost 15 years ago!!!! As you can imagine the manual has disappeared into oblivion.

Let me fully digest all this info and see if I have more questions laster. You started to clear the fog, so no worries about further confusion.

Brian

 

[New Hampshire]

Alright, got another question of DOF. I was toying around and doing a little DOF "hands on" practice to help me better understand the guage and how to get the various levels of sharpness/bluriness in the background/foreground (no film in the camera, just dry running it to help me cement things in my mind.) While doing so I remembered something.....I have a circular polarizer on. I checked the packeage it came in and it says the polarizer has a 1 2/3 to 2 stop value (my lens only doe 1/2 stops so I assume they mean 2.) So here is the big question:

How does the filter affect DOF?

See, the reason I ask is because if DOF is directly affected by the f/stop, would not having to adjust 2 stops to compensate for the filter thus change your DOF? Example (to use Stephs as a basis): Without polarizer you get a depth of field from say .8 meters to 5 meters at f/22 which gives you the exact DOF you want. Now add the polarizer and you must compensate 2 stops for this by going to f/11. But this changes your DOF right? Or, could I do some fidgiting around with the shutter speed and the TTL light meter?

Brian

 

[markc]

The polarizer doesn't change the aperture, only the amount of light reaching the film. It's like shooting through sunglasses or the like.

Changing the shutter speed from 1/125 to 1/500 is also changing by one stop, just not one f-stop.

 

[New Hampshire]

The polarizer doesn't change the aperture, only the amount of light reaching the film. It's like shooting through sunglasses or the like.

Changing the shutter speed from 1/125 to 1/500 is also changing by one stop, just not one f-stop.

Thought so. Thanks, you all have been absolutely awesome!

Brian