A Pea on My Lens?

For everyone who thinks a speck of dust or dirt on their camera lens will affect the quality of their pictures, try this trick, that a photographer recently shared with me:

Tear off a piece of the sticky part of a "Post-It" note about the size of a pea. Place that piece of paper directly on your camera lens (don't worry it won't hurt it). Now look through the camera lens and what do you see?

It's amazing!

Examples?

No thanks, I don't want any kind of adhesive on my lenses.

Yes we hear this one all the time. People who don't use protective filters actually quote this as a justification.

The real problem with dust inside or worse scratches on elements is the increased level of light dispersal resulting in lowered flare resistance even though it may make minimal difference to sharpness.

It’s not a good idea to get even mild adhesives on optical coatings as they may damage it.

the point is, the lens sees right thru it. i can stick a hand up in front of my 70-200 as i look through the viewfinder. as long as you are focusing past it, the item will disappear.

you can shoot thru chain-link fences and stuff like that too.

MikeMcL wrote in post #2376083
the point is, the lens sees right thru it. i can stick a hand up in front of my 70-200 as i look through the viewfinder. as long as you are focusing past it, the item will disappear.

you can shoot thru chain-link fences and stuff like that too.

Thats why you want a 200 f2.8 for the zoo, nothing wrong with that.

You will note the chain link fence lowers the image contrast, sometimes significantly, although it is completly blured out.

Lester Wareham wrote in post #2376096
Thats why you want a 200 f2.8 for the zoo, nothing wrong with that.

Here we go...can't put down a good lens(e), eh Lester ?
So, being and engineer and all that, let us take a closer look at this phenomenon. One pea makes no difference, right ?
How about 3 peas ? Nothing ?
How's 5 peas ?
Let us go to 10 peas ? See nothing yet ?
Now, a really bad one.....15 peas ! Ooops.....that's no good, the image is all blurry !

So, what do we have here? Some kind of an optical step function ? No effect up to 14 peas. However, the 15th pea abruptly turns the image fecal ?

Anybody read "emperor of ocean park?" Part of the plot turns on whether two dark spots in an autopsy photo are bullets or dust on the lens. Hah !!! One of two plot errors I've seen in an otherwise excellent book.

The invisibility of the spot-on-the-lens is because the spot is so far out of focus that it's circle of confusion is larger than the image. Nonetheless, the image of the spot is indeed spread out over the entire sensor, resulting in a loss of contrast. As petkal drools more pea soup onto his lenses, the fraction of the image on the sensor from out-of-focus objects on the lens increases, and image quality goes down.

Mike and Lester are quite right - a lens with narrow depth-of-field can shoot thru a close chain link fence. You just get a loss of contrast, due to the blurred out contribution from the fence.

Peter, a better nursing attendant would mop up the pea-soup before the drool departs from your chin, helping to keep your lenses clean. Look into it.

An item on the element may not be visible, per se, in a photo. But an item placed into the optical path will cause reduction in the lens' sharpness! There is a 'soft focus' portrait lens (I forget if medium format or large format lens) that puts something into the optical path to deliberately degrade the sharpness (so that ladies do not object so much to the wrinkle lines that are too apparent with a conventional lens!

I only stick Beans on my front element seems to give better contrast then pea's
Rob .

PetKal wrote in post #2376129
Here we go...can't put down a good lens(e), eh Lester ?
So, being and engineer and all that, let us take a closer look at this phenomenon. One pea makes no difference, right ?
How about 3 peas ? Nothing ?
How's 5 peas ?
Let us go to 10 peas ? See nothing yet ?
Now, a really bad one.....15 peas ! Ooops.....that's no good, the image is all blurry !

So, what do we have here? Some kind of an optical step function ? No effect up to 14 peas. However, the 15th pea abruptly turns the image fecal ?

Hooray, someone who does not just believe something without applying some thought.

A pleasure to share the web with you sir!

So what have we learned here from this little back-of-the-envelope finite element analysis ? Nothing that was not discovered some 2500 years ago !Except, at that time they used grains of rice or something.

However, we have made some discoveries which are probably peripheral to the issue, viz. Gyrob is into beans and DavidEB is gerontophobic.

gyrob wrote in post #2376332
I only stick Beans on my front element seems to give better contrast then pea's
Rob .

Hot, tip. Does it work with all beans or will any do?

Lester Wareham wrote in post #2376449
A pleasure to share the web with you sir!

Well, the pleasure is all mine !

I'll stick with my L lenses. I'll leave pea lenses to the vegetarians.