View Full Version : Larger Aperture = Smaller DOF...How?

12-28-2005, 01:55 PM
It's common knowledge, as evidenced by Fred Miller and his 50 f1.8, that larger apertures yeild a very shallow depth of field.

I know that this happens, but I'm curious as to what actually causes it. What is it about letting more light in that blurs the background (and foreground)?

Just something I've been thinking about. :)


12-28-2005, 04:37 PM
wow, Brian, I think I might have to go home and dust off my optics texts. :lol:

I've taken Geometric, Physiologic and Ophthalmic optics and can tell you directly how they relate to your eyes, but the concept of depth of field (DOF) complicates it b/c when we see, wee speak in terms, usually, of clear and blurry!

Its simple and yet a bit difficult to explain. (Damn, the DOF!) please forgive me if this sounds so basic:

an aperature is a hole.

So, the larger the hole, the smaller the depth of field
the smaller the hole, the larger and longer the depth of field.

That is the inverse relationship...now, WHY that is is a bit tougher to explain. Next time I see you, I can show you with ray diagrams why that is.

But seriously, I can probably bettter explain it on a phone call so dont hesistate to call me.

now, I did a quick search and reviewed this page below. Scroll through it and you'll find a section on DOF and it illustrates it with some ray diagrams. But, in all honestly, I doubt it might do a great job of explaining WHY that inverse relationship exists( and I think that is what you are asking, right).


Oh, actually, I just realized the most basic reason... When we as eye docs examine the eye and how the eye process optics, we need to study light as 2 entities. We look at it from a wavefront model and also from a light ray/photon model. So that's why I ve been referring to ray diagrams.

We have to look at it both ways b/c if you look at optics in only wavefront or light ray terms, certain aspects cant be understood but either singly. Accepting both yields understanding.

And BTW, you might here wavefront lasik in the media these days? All relates to basic optics.

ok, Im done rambling, so give me a call if I've only confused you more!

12-28-2005, 11:44 PM
Mario is right, but there is more to it and there will be more than what I say....

first a link to confuse you more

Ok, If I have three lens's at F5.6

35MM -------------------- 50MM------------------300MM
nice wide Depth---------Getting narrow---------LOTS OF BACK-ROUND BLUR

Ok, all the above at the same Aperture of 5.6 will give you different DOF (Depth of Fields)..... Now some of you are saying, "I do not give a flying [email protected]&, HOW DOES THIS HELP IN AVIATION?

If you are shooting lets say a 500MM Canon F4 lens, you need to have AI servo focusing so the camera' auto-focus will track the aircraft. If you hold the shutter down to get a focus lock and then take the picture, the focus band is so narrow that the aircraft flew out of the sweet-spot and you now have a soft image. If you were shooting 300MM at F4, you still need to be Very FAST and the tail of the aircraft will probably be sharper than your nose. You start shooting in the higher fstops to keep the plane in focus. (Aggadurated a little bit but you get the drift)

The bigger the zoom, and the wider the aperture the more blur....if you start shooting F-20, you would get like 3 feet to 500FT in focus....hard to take an out of focus shot...but your speed will be tripod worthy unless on the sun shooting 1600ISO! Your F-stop will give you the effect you want or do not want in all shots!

The Wide aperture of my little 50MM 1.8F II is wild because I can take shots at night handheld.....That night the camera was not opened up to 1.8 and was not at 1600 so most shots are SOFT as hell. To see some shots on what the Lens really does at night, LOOK HERE at any shot with people in it!

The problem is at F1.8, If you take a picture of a couple, they need to be completely the same distance from you because more likely one will be out of focus because of the inch or so DOF!!!!!

If you are shooting sports at night, with lets say a 70-200MM f4-5.6, the shots will come out like ****...if you use a 200MM f-2 lens, they will be fast and great...but the lens is like $1500 more and will be like 10 times the size and weight. Big glass that allows the Low aperture will be FAST and expensive ALWAYS. It is nice getting 30th of a second where my kit lens give me a 3 second exposure! Speed is everything, a fast lens is a must and the 50mm-100mm is perfect for portrait shots.

The Canon 50MM 1.8 II lens is my best lens, the 50MM gives you NO DISTORTION and being a non zoom or "PRIME LENS" it is like having an L lens for UNDER $100.00. Most people feel that the 50MM to 80MM is the perfect portrait lens. I have it on my camera all the time. At night or inside, I can shoot all around the house with no flash and have perfect shots without strobing people.

I hate Flashes, I hate announcing that "I JUST TOOK YOUR PICTURE". I hate to see the room look my direction, I HATE FLASHES and a nice 1.8 is perfect for nice indoor shots with great quality and NO FLASH. I HATE FLASHES!

With that said, I just got a great new Sigma Flash for Christmas, it will figure out what lens, the distance of the object (BY YOUR CAMERA"S ZOOM) and look at if you have the flash pointed in the air and at what angle (If you are strobing the ceiling for a NICE FLASH SHOT) and it will set the camera and flash up for the exact settings....I can also use it on a stand and my camera flash will fire it off from the visual strobe so the flash will act like a wireless slave flash.......but the best part is that I can use my camera at 1/4000ths of a second unlike almost all other flashes and the built in flash that is only capable of 1/200ths of a second.

Sorry about the Babble, I have enough drinks in me to kill a horse!!

12-29-2005, 09:31 AM
Bri, just browsed some notes last night and to be truthful, I dont want to overcomplicate your original question as I may already have:

but I'm curious as to what actually causes it. What is it about letting more light in that blurs the background (and foreground)?

after reviewing, perhaps this is a bit more clear... remember that inverse relationship I noted originally. That is a law. If all other things are held constant, as the hole(aperature) gets larger, the DOF decreases and vice versa.

But, WHY this occurs, again, deals with the natural properties of light in that it behaves as light rays but also as electromagnetic radiation in the form of wavefronts.

also, you are not really letting in more light...actually, you are allowing more light rays and they have a tendency to bend a bit around an aperature. When I think of more light, the term is luminance.

I am trying to explain it while avoiding shop talk of F stops, etc.

quick example: you have a dimmable lightbulb in a room with the door closed. the bulb is half lit. if you open the door, are you letting more light in/out? No, you are allowing more lightrays to escape (you've opened an aperature). More light (luminance) would entail you turning the lightbulb up to full lit. Does that make sense?

That is the most basic answer. After that, my discussion and Fred's only pertain to examples. But the underlying principles lies in understanding light in terms of wavefront and ray diagrams...and thats 9 months of optics, bud. Glad thats over.

Its very easy for me to explain with respect to eyes though since the eye is a camera with a shutter of infinite speed and a variable aperature(your pupils!).

12-31-2005, 10:48 PM
Okay, I now know for a fact that I've gotten in over my head. And that's prettymuch ALL I know. :)

I've been familiar with the inverse relationship between aperture and DOF for a long time now, but it looks like explaining it involves the actual behavior of "light rays" and "electromagnetic radiation" in the presence of a small or wide opening. Not something I'm likely to understand through a few posts here on NYCA.

I'm taking physics right now and the subject of light is coming up in the curriculum, so I suppose I'll touch base with you guys again when I have more of a foundation in all this.

Mario, maybe we'll have some time to discuss this when I come in with a piece of hair in my puncta. :)

Fred, I think someone's calling you over at FRG. Go! Run! ;)

Anyway, thanks to the two of you for taking a stab at explaining this. Happy New Year!


01-02-2006, 07:59 PM
OK, I will simplify it to an extreme.....

Put a pinhole in the top of a box and in a dark room put some film on the inside bottom of a box. You have what is called a pinhole camera. There really is no depth of field....the hole is so small that everything will appear in focus when the film is developed, the problem is that the little hole is letting so LITTLE light it will take a while for the picture to develop the film inside the closed box. cover the hole to stop taking the picture.

1. small hole, everything in focus...there was no lens used...just a pinhole.

The wider the hole, the more precise optics (Lens) and more precise the angles of this needs to be. Lens's with wider holes will be Heavier, Bigger and will let more light in....then you need to worry about less depth of field. There really is not much more to it....if you have a 50mm with a pinhole, everything is in focus....if you have one at 1.8, it is a huge hole that is perfectly focusing the light to hit the CCD CMOS or film at the exact distance........Next time you look at a magnifying glass, you will see how a very small distance change on a big piece of glass will make a huge difference...

I can not post a picture here right now but I hope this helps.

Now change the angles of the rays on the top and bottom as is the lens was the size of pinhole....it will be like two almost straight lines.

01-02-2006, 08:13 PM
fred, hows that explain the inverse relationship? That is what Brian is asking about.

ps a parrot just **** on Tommy's car, and it resembles an L-1011.

01-03-2006, 10:07 AM
good talking again Fred.

Like I said, no biggie. Its really just simply applied physics (in form of geometric optics) and math equations.

I am looking back at 10 weeks of optics in horror. How I loathed Geo'!