Depth of field question help

Ask about gear, cameras, lenses, technique or lighting

Moderators: tswinner, bvanant

Postby TheWetRookie » Sun Aug 08, 2010 1:05 pm

Hi All,

I have a question in regards to depth of field. I understand that with the changing of the aperature settings, you can change the depth of field and what will be in focus and what won't be based upon how much light is being let in. (I hope I am correct here)

My question though is why does the depth of field change with the amount of light that is being let in and what will be in focus and what wont? My thinking is that it is either in focus or out of focus but not both at the same time and variable.

Thanks

Mike
Mike B.
Cold water diving at it's best on the West Coast of Canada. Tons of life and diving here. -
Canon Powershot A620 with a Ikelite housing and sea life strobe
User avatar
TheWetRookie
 
Posts: 17
Joined: Sat Aug 07, 2010 10:26 am

Postby bvanant » Sun Aug 08, 2010 1:39 pm

It does not depend on how much light you let in but it does depend on how big the aperture of your lens is. You can control how much light gets into your picture by either changing the shutter speed or changing the f stop. If you are using a strobe that is faster than your shutter speed though, then you will only have the fstop to play with.

A perfect lens should only be able to focus at one distance but in reality, lenses have something called the circle of confusion. If you focus a lens onto a flat surface, a perfect lens would have a very tiny spot. Real lenses have bigger spots on the flat surface and the size of the spot that isn't blurry is called the circle of confusion.The theory behind all this is a bit complex but the practical use is quite simple.

The longer the focal length of the lens, the shallower the DOF appears, but that is more due to the fact that telephoto lenses are set up to shoot small things far away.

The smaller the sensor, the larger DOF you get, which is why P&S cameras aren't exceptionally good for making shallow DOF exposures.
Finally fstop. Large fstops (small numbers) mean shallow DOF, while smaller ftsops make more DOF. To calculate DOF for a particular camera/lens look at
dofmaster.com

Bill

Bill Van Antwerp Canon/Nauticam/Subal/Inon Lots of glass


Technical Advisor to Bluewater Photo


http://www.blueviews.net

User avatar
bvanant
 
Posts: 309
Joined: Wed May 12, 2010 4:16 pm
Location: Los Angeles (more or less)

Postby TheWetRookie » Sun Aug 08, 2010 3:19 pm

Thanks for the help but I am still unsure about things. Let me try and rephrase my question.

Aperture controls the bokeh, or blur, of the background. A large aperture such as F2.8 will heavily blur the background. A very small aperture will keep the background almost in focus.
From the tutorials section.

Example: I have a picture in focus, but by changing the f-stops I can change what is in focus around it, the background. I understand that I can do this but I don't understand why this happens. Why, when I close the aperature, the surroundings are in focus but when Iopen it, less surroundings are in focus yet I am not adjusting the lense?

Hopefully this helps with understanding my problem/thinking. Maybe I am overthinking this too much and just need to accept that it can be done. I am a poor simple man with a simple mind ;).

Thanks
Mike B.
Cold water diving at it's best on the West Coast of Canada. Tons of life and diving here. -
Canon Powershot A620 with a Ikelite housing and sea life strobe
User avatar
TheWetRookie
 
Posts: 17
Joined: Sat Aug 07, 2010 10:26 am

Postby douglasjhoffman » Sun Aug 08, 2010 5:16 pm

Hows this. Keep it simple. As you stated you know you can achieve sharp backgrounds with F22 and images with blurred backgrounds at F2.8. Do not take the fun out u/w water photography by trying to understand all the why's. The technical side is exactly that technical. Many people including me have a hard time with these concepts. The good news is that by repetition you will learn the difference. Start with what works and eventually work your way out of your comfort zone and establish your own style. As you gain experience the answer to your question will become evident. One day it will just hit you. In the mean time have fun and keep shooting.


When creating images underwater think not only about the technical side but about the creative and esthetic side. If your photographing a wide angle seen with a turtle. Do you want the reef or water column behind it crisp or soft. Knowing what you want the image to like like will help you choose the right DOF.


Have fun and post your images. There is a lot of talented people willing to share.
User avatar
douglasjhoffman
 
Posts: 57
Joined: Fri May 14, 2010 1:07 pm
Location: Maui - world wide

Postby James » Sun Aug 08, 2010 5:45 pm

I think you are confusing yourself.

The DOF is optical physics at play. For any given FL, focal length, increasing the diameter of the clear lens opening, aperture, the the DOF will decrease and light transmittance will increase. There is a fairly complex equation that relates all of this and your not really going to make me dig it up are you?

Also, for any given FL and set aperture, the DOF increases as the distance to the subject increases.

Optical physics can be a bit daunting for us laymen, telescopes work the same as camera lenses, work the same as binoculars etc.

The focal length of a lens is the distance to which light can be focused to a point.

The (clear) aperture of a lens is the diameter of the lens that light can enter.

The f stop is a ratio of the FL to the aperture.

For example, a telescope has a 1,000mm FL. The clear aperture is 100mm, the operating f stop of the telescope system is therefore f10.0 .

A camera lens has a variable aperture controlled by an iris or diaphragm so it can be a bit complicated if we must pull equations out of the hat to describe the relationships.

Some cameras including the G11 can artificially impose a greater f stop on the lens system by inserting a neutral density filter into the system, obviously, this cannot have any effect on the DOF.

Every time you double your shutter speed, you decrease by 1/2 the amount of light reaching the camera film/sensor.

Every time you adjust your lens aperture a full stop, you either double or half the amount of light reaching the film/sensor.

Every time you adjust the light output (knob) on your strobe by a stop you are a doubling or halving the effective light output but with a strobe the brightness remains the same, the exposure is effected by increasing or decreasing the pulse length.

Example, these are the same exposure:

(shutter speed 1/125, f 8) and (shutter speed 1/250, f 5.6)

The f stop scale is a log scale. Each step up or down below is a full stop and will double or half the light transmittance.

1/ 1.4 / 2 / 2.8 / 4 / 5.6 / 8 / 11 / 16 / 22 / 32

James
Last edited by James on Sun Aug 08, 2010 5:57 pm, edited 1 time in total.
Swim down, swim around, swim back up

Canon S90 with FIX90 housing, 2X Inon D2000 strobes, DIY tray, Inon UFL165AD, Inon UCL165AD, Inon UWL100-67 with dome kit, ordered Fisheye UWL-04
User avatar
James
 
Posts: 139
Joined: Thu May 13, 2010 7:18 am

Postby James » Sun Aug 08, 2010 5:53 pm

Do you need to know the how of all this, not really, but you do need to understand the effect of each so that as you manipulate your camera settings, strobe settings, filters etc to accomplish an effect, that you understand what the effect will be instead of a WAG.

James
Swim down, swim around, swim back up

Canon S90 with FIX90 housing, 2X Inon D2000 strobes, DIY tray, Inon UFL165AD, Inon UCL165AD, Inon UWL100-67 with dome kit, ordered Fisheye UWL-04
User avatar
James
 
Posts: 139
Joined: Thu May 13, 2010 7:18 am

Postby bvanant » Sun Aug 08, 2010 6:26 pm

As James points out, the exact details or why are not too important. Remember that DOF is really the range of distances reproduced in an image over which the image is not unacceptably less sharp than the sharpest part of the image. (from Bob Atkins). This means that DOF is related to how you are looking at the image, how big the image is, how far away you are etc. It is not an inherent parameter for any given lens, although Bokeh is.




For James:

Image
Here F is the focal length. D is the subject distance, c is the circle of confusion and fn is the f# (f-stop) of the lens. Now this equation doesn't reduce to some simple rule of thumb. However we can say that over the range of focus distances which aren't in the macro range (where D is close to F) and which aren't close to the hyperfocal distance (where D = F*F/fn*c) you can "guesstimate" that the depth of field ratio between two lenses used at the same aperture and focused at the same distance by assuming it's proportional to the size of the circle of confusion and inversely proportional to the square of the focal length (From Bob Atkins)

Bill

Bill Van Antwerp Canon/Nauticam/Subal/Inon Lots of glass


Technical Advisor to Bluewater Photo


http://www.blueviews.net

User avatar
bvanant
 
Posts: 309
Joined: Wed May 12, 2010 4:16 pm
Location: Los Angeles (more or less)

Postby bvanant » Sun Aug 08, 2010 6:33 pm

TheWetRookie wrote:Thanks for the help but I am still unsure about things. Let me try and rephrase my question.

Aperture controls the bokeh, or blur, of the background. A large aperture such as F2.8 will heavily blur the background. A very small aperture will keep the background almost in focus.
From the tutorials section.

Example: I have a picture in focus, but by changing the f-stops I can change what is in focus around it, the background. I understand that I can do this but I don't understand why this happens. Why, when I close the aperature, the surroundings are in focus but when Iopen it, less surroundings are in focus yet I am not adjusting the lense?

Hopefully this helps with understanding my problem/thinking. Maybe I am overthinking this too much and just need to accept that it can be done. I am a poor simple man with a simple mind ;).

Thanks


Actually Bokeh is not the blur of the background but rather how pleasing or not pleasing the blur can look. Bokeh is a lens property i.e. there are some lenses with wonderful Bokeh and others of the same focal length that have terrible Bokeh.

Bill

Bill Van Antwerp Canon/Nauticam/Subal/Inon Lots of glass


Technical Advisor to Bluewater Photo


http://www.blueviews.net

User avatar
bvanant
 
Posts: 309
Joined: Wed May 12, 2010 4:16 pm
Location: Los Angeles (more or less)

Postby TheWetRookie » Sun Aug 08, 2010 8:43 pm

Excellent, clear as mud ;) ;)

I will stick to the simple answer, it can be done. Go diving and have fun doing it and not worry about all the why's.

Thank you all for your help and sorry for making you type so much though it was greatly appreciated.

I shall go out and practice now.
Mike B.
Cold water diving at it's best on the West Coast of Canada. Tons of life and diving here. -
Canon Powershot A620 with a Ikelite housing and sea life strobe
User avatar
TheWetRookie
 
Posts: 17
Joined: Sat Aug 07, 2010 10:26 am

Postby Vizart » Mon Aug 09, 2010 6:44 am

Let’s try this simple, non nerdy, approach for a second, go in your virtual kitchen and open the faucet fully, fill up a glass and clock it, let’s say that it filled up in 1 second, now close it in 1/2 and it will now take 2 seconds to fill said glass, now close it to a ¼ and it takes 4 seconds and so on… so that’s the relationship between aperture (the faucet which control the amount of light passing through is aperture) and the shutter speed (the clock that control how long the light will go through the aperture).

As for depth of field, let’s take another example we are familiar with; a light source.
If you have a flood light you have a large aperture, broad and soft.
If you have a spotlight, then you have the small aperture, narrow and sharp.
When you close down the aperture, you narrow and sharpen the cone of light going through the lens, thus sharper details appear to be more visible, open it and you flood and soften details.

Lastly, aperture value can be confusing mainly because we use small number to describe large aperture and large number to describe small aperture, a simple memory helper is to think in negative terms, at f/4 you have four time less light, at f/16 you have 16 time less light etc. this is just to guide to remind you in which direction to go.

I understand that I won’t be winning scientific accolade for this description, but it works in my class room and people usually get the picture.

And here is one for the road, If I tell you that I have a 50mm f/2 lens, that mean I have an optical tube of 50mm long but with a maximum internal passage way of only 25mm (f/2), if I tell you to that I am using this lens at f/8 that means I am reducing the passage way (aperture) of this lens all the way to 6.25mm.

From this we can simply deduct that a 100mm f/2 will have a maximum opening for letting the light in of 50mm, the 28mm will have a 14mm, so aperture in itself is not a size unless its in relation to a lens.
,
Jean Bruneau, Aquatica Technical Adviser
3025 De Baene
Montreal, Quebec
Canada, H4S 1K8

jean@aquatica.ca

514-737-9481
User avatar
Vizart
 
Posts: 32
Joined: Thu Jul 08, 2010 11:12 am

Next

Return to Underwater Photography Questions

Who is online

Users browsing this forum: No registered users and 5 guests

cron