Depth of field
This report covers:
- What is depth of field?
- Let me show you
- Focus remained the same
- No jargon
- The deal
- A test
Today I’m addressing a question that came in about a week ago concerning depth of field. We were talking about peep sights in general in Part 2 of the Crosman Precision Diopter Sight System. I had said they can increase the depth of field for you. Here are some comments we got that led to this report.
|Decksniper BB People’s eyes can differ but for most folks does the 10 meter blinder or in your case the rubber cup affect sharpness of the front sight at 10 meters or even 25 yards? Or does it just help the rear peep do it’s thing, which is to bring both the front sight and target into clear focus? I ask this because with my astigmatism a peep dramatically helps me even on a pistol and a lens peep can sharpen a blurry red/green dot optic. Deck|
|Tom Gaylord (B.B. Pelletier) Deck, When you stop the light down with the black eyecup you increase the depth of field. I will try to show this in a report. BB|
|Decksniper BB Oh boy! Looking forward to it. Thank you,|
I would be interested seeing how you show that also. The depth of field. Gunfun1
I’ve done it before with a film camera. Digital cameras are so sophisticated; I hope I can do it with them.
Ok hope so too. Will be waiting.
What is depth of field?
Depth of field is the range of distances at which things are in sharp focus. It’s easy to demonstrate with a film camera that has all manual controls. But with a modern digital camera it’s much harder to achieve because the camera acts like your brain. It optimizes everything, the light, the focus and the length of time at which the image is exposed and some other things that will come up in this report.
However, BB Pelletier, being the boss clown of this outfit, put on his big girl panties and set to the task. I could have done a little better if this was all that was on my plate, but, like all of you, I’m a very busy boy.
Let me show you
They say a picture is worth a thousand words, so here come three that illustrate my point.
I was just fooling around, trying to see what I could do to illustrate what depth of field is when I took this picture. The numbers 65, 368 and 396 are in focus. The numbers closer to the camera and farther away are out of focus. This is a narrow depth of field.
To increase the depth of field you need to do several things. Reduce the size of the hole through the camera lens, and increase the exposure time to allow the image to resolve. In a camera this is done by selecting the smallest exposure hole you can get with the lens that’s on the camera. If you have a film camera you need to consider the “speed” of the film, which means how fast it exposes in light to create an image. That is done by holding the shutter open long enough for the light coming through the lens to react with the film.
When you see, your brain does all of this for you — it sets the speed of the “film”, opens the hole in the “lens” and corrects the exposure in all other ways. The thing is, you don’t usually see the world in still photos — you see video. So your brain is working fast and sliding parameters around to keep the images sharp. Focus on something closer or farther away and your brain takes care of everything. To you the world looks “normal.”
A digital camera does many of those same things for you unless you override its automated features. And even then there are some things that cannot be overridden. The camera wants your pictures to be in focus. Usually that’s what I want, as well, but not for this report. So I had to learn how to overcome as many of the automated background features as possible to get what I wanted.
By opening the hole through the lens wide and speeding up the digital “film” I forced the camera to “see” a narrow depth of field.
By making the hole smaller through the lens I increased the depth of field. I also decreased the speed of the digital “film” and exposed this image much longer.
Focus remained the same
The focus for both pictures was the same. What made the depth of field increase in the lower picture were the things mentioned in the caption for that picture.
For both pictures shown above the camera was in the same position. All that changed are the things mentioned in the caption of the previous picture.
I have purposely avoided as much jargon as possible in this report. Talking about things like ASA/ISO, shutter speed and f-stops only serves to confuse people, so I avoided them. Photographers speak in such terms, but they are talking about the same things I have mentioned in this report.
Okay, I’m changing the topic a little, though what I’m about to say relates to what’s been said so far. What does a blinder do? It allows you to keep your non-sighting eye open while sighting through a small peephole. I have read where people have said that the blinder allows more light to come into your eyes and that is what helps you sight. That, they say, is why a light-colored translucent blinder is better than a dark blinder. BB says, “Hogwash! It ain’t true.”
What the blinder does is prevent a second image from confusing you when you are trying to see the front sight and target through a small peephole. Why a small peephole, you ask? Because that reduces the amount light coming through to that eye, causing your brain to adjust the pupil of your eye to expose the image over a longer period of time and THAT increases your depth of field! Now that you understand what depth of field is, you realize how a smaller peephole lets both the front sight and the target to be in relatively sharp focus.
Why not just close your non-sighting eye in a prolonged wink? Here is the deal. If you squint your non-sighting eye you also squint your sighting eye. Some are better at controlling this, but we all have this tendency. What it does is cause the peephole to let less light pass through to your sighting eye. While that sounds like a good thing, it’s not. The problem is, your sighting eye opens and closes just a little throwing off the light coming through and messing up your sight picture.
Take a small card — business card size — and punch a hole in it. The hole does not have to be perfectly round. You can punch it with a ballpoint pen if you like. Hold the hole up to your sighting eye and try to close your other eye by winking. You will see that the hole gets smaller and darker around its edges as you do this. That’s what squinting does when you shoot with a peep sight. Not good!
I hope this report has helped you understand what depth of field is and how it comes about. If you use peep sights, this is why using them correctly matters so much.
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