Can You Explain Something to Me?

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Dan Krull
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Can You Explain Something to Me?

Post by Dan Krull » July 20th, 2010, 9:45 am

Please be as technically detailed as possible and explain to me why when I shoot a shot with no flash, only portions of the frame are in focus, but when I shoot with the flash the whole field is in focus.

Examples

Without Flash:
Image
With Flash
Image

Thanks in advance.

Dan

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justinm
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Re: Can You Explain Something to Me?

Post by justinm » July 20th, 2010, 11:32 am

Flash usually always allows more detail. I don't know the science behind it I just know you need to use it. So basically 60% of the time it works every time.

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MattSullivan
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Re: Can You Explain Something to Me?

Post by MattSullivan » July 20th, 2010, 12:01 pm

idk the science of it either. but built in flashes sometimes make for really 'flat' shots. having just part of the animal in focus, such as the head, at least from a photography standpoint usually makes a more 'interesting' image. IMHO

beautiful snake and nice shots btw :thumb:

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Chad M. Lane
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Re: Can You Explain Something to Me?

Post by Chad M. Lane » July 20th, 2010, 12:06 pm

Was your F/stop the same in both shots? If you give us more exif data I think you'll end up getting a better answer.



Thanks,
Chad

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MHollanders
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Re: Can You Explain Something to Me?

Post by MHollanders » July 20th, 2010, 12:12 pm

Do you shoot the camera with auto exposure when using flash? Depth of field (basically, the amount of areas that are in focus in your shot) has to do with your aperture. The higher the number, the smaller the aperture and thus the higher DOF. You can afford to use a small aperture when using flash because you're utilizing an external light source (the flash). However, if you're shooting with natural light, you need a larger aperture to compensate for the lower light, thus decreasing your DOF. Hopefully this helped.

Edit: FWIW, I prefer the first shot. ;)

Later, Matt

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umop apisdn
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Re: Can You Explain Something to Me?

Post by umop apisdn » July 20th, 2010, 4:04 pm

It would help to know more technical detail about what you're shooting with and how.

If you're shooting in some sort of automatic mode, the camera is compensating the aperture to some extent, which is affecting your depth of field. Basically, if you kept all of the settings in the first shot the same, and simply added flash, your picture would be overexposed. Had you used a higher f-stop, slower shutter speed, and a tripod, you would have been able to achieve a greater depth of field in your naturally lit shot.

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Re: Can You Explain Something to Me?

Post by milmoejoe » July 21st, 2010, 3:01 am

Not positive, but it looks like your settings changed from shot 1 to shot 2. As Chad mentioned, you really need to explain the technical foundation (or at least leave the metadata intact) for others to figure out what went wrong.

Your first shot is underexposed and essentially selective focused / subject not aligned with the plane of the sensor. Some folks do this intentionally, lots of times it's done in error.

Your second shot appears to be manually metered and uses flash as a main light source, stopped down, etc.

If you set the camera on a tripod, Aperture priority mode, f/8, ISO 400, one with flash (fill), one without, your depth of field will be the same.

The scenario changes when you manually meter, add great amounts of light, and stop down to a tiny aperture as it appears you have.

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Dan Krull
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Re: Can You Explain Something to Me?

Post by Dan Krull » July 21st, 2010, 10:54 am

Much good info.

Ok. So I shoot with a Nikon D100. I keep it set to Aperture Priority at all times. I have the white balance set to auto and the quality set to fine, and I never mess with the ISO, because I'm not sure what it does. :roll: I thought it had something to do with movement in the shot or something.
I adjust the Fstop to fit the light levels.

So to answer your questions, or to confirm your suspicions as the case may be, I did adjust the Fstop from 1 shot to the next, and yes the second shot would have been over exposed had I not.

I prefer the first shot too because the colors are rich and more like the actual snake, but I like SOME shots to be in full focus for various reasons. I'm fine, and not a serious enough photographer, with just shooting a couple of shots with the flash to get the id photo with total depth of field every time I shoot, but if there is a way for me to use the natural lighting and get a fully focused shot, I'd like to hear about it.

I KNOW I SHOULD JUST TAKE A CLASS. I've been teaching myself slowly how to use this camera with the help of people like you guys, because I don't have time to read the manual or take a class. So, thanks again.

Dan

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MHollanders
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Re: Can You Explain Something to Me?

Post by MHollanders » July 21st, 2010, 2:14 pm

Yeah, the different f stops changed the DOF in the two pics.

Later, Matt

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Dan Krull
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Re: Can You Explain Something to Me?

Post by Dan Krull » July 21st, 2010, 3:25 pm

So it is the F stop and not the amount of light that effects the depth of field? I see.

DAN

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Chad M. Lane
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Re: Can You Explain Something to Me?

Post by Chad M. Lane » July 21st, 2010, 3:55 pm

The smaller the aperture (the larger F/stop number) the greater depth of field you'll get. The larger the aperture (smaller F/stop number) the less depth of field you'll get. Though you don't want to go to high of a F/stop as some lens/camera will actually lose sharpness at extreme F/stop (depending on lens/camera combo).

Focal plain also plays a role, subject vs sensor placement. Also the mm of a lens changes the depth of field in small amounts.

I hope this helps a bit.



Cheers,
Chad

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Re: Can You Explain Something to Me?

Post by Erik Williams » July 21st, 2010, 5:45 pm

Chad M. Lane wrote: Also the mm of a lens changes the depth of field in small amounts.
This is a good quote because it is only a SMALL amount. The primary considerations in depth of field are magnification and F/stop. A subject at 1/10 life size, same f/stop will have very similar DoF at 400mm or 24mm. However, 1/10 life size for a 400mm lens might be four feet from the lens, and for a 24mm it might be touching the lens element (I made those numbers up!). At the same distance from the sensor, the lenses will boast different DoF at the same F/stop, because the magnification is different.

Many sources will tell you that a longer focal length has less DoF, but it's a mis-interpretation of the relationship between magnification and focal length. At a similar magnification, all lenses will perform similarly. There is a small, variable difference between wide angle and telephoto, but it's insignificant to photographers.

Except when the magnification occurs close to hyper-focal distance. Once Hyper-focal distance is part of the equation, you can assume that a lens close to hyper-focal distance will always have MUCH more DoF than a lens that is not near hyper-focal distance.

E

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Chad M. Lane
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Re: Can You Explain Something to Me?

Post by Chad M. Lane » July 21st, 2010, 6:06 pm

Yes it's a very small amount.

Focal length 20mm, Focus distance (m) 0.5, Depth of field (m) 0.482
Focal length 400mm, Focus distance (m) 20, Depth of field (m) 0.404


So yes it only effect it in a VERY SMALL amount. It also will vary in different 400mm lenses, and camera bodies. Again only in SMALL amounts.



Cheers,
Chad

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chrish
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Re: Can You Explain Something to Me?

Post by chrish » July 24th, 2010, 8:43 am

Herp Ninja wrote:I adjust the Fstop to fit the light levels.........I did adjust the Fstop from 1 shot to the next, and yes the second shot would have been over exposed had I not.
It shouldn't have been overexposed. Your camera should have adjusted the exposure to which ever f/stop you used. If you are in Aperture priority, the camera adjusts the shutter speed (and sometimes the ISO if you have it on Auto) to provide enough light. In theory, a non-flashed photo should be just as "bright" at f/2.8 as it is at f/22 using aperture priority. With flash it can be different because the flash may or may not be powerful enough to provide enough light at f/22.
I prefer the first shot too because the colors are rich and more like the actual snake, but I like SOME shots to be in full focus for various reasons. I'm fine, and not a serious enough photographer, with just shooting a couple of shots with the flash to get the id photo with total depth of field every time I shoot, but if there is a way for me to use the natural lighting and get a fully focused shot, I'd like to hear about it.
Of course you can get a fully focused shot (aka adequate depth of field) using natural light, it just takes a longer exposure (slower shutter speed). That is OK in some cases, but if the animal moves you get a blurry shot (see below for explanation).
I KNOW I SHOULD JUST TAKE A CLASS. I've been teaching myself slowly how to use this camera with the help of people like you guys, because I don't have time to read the manual or take a class. So, thanks again.
You really don't need a class. What you need is to understand the basics of exposure and then go out and shoot a lot of pictures playing with the information.

The basic principle is simple:

1. It takes a certain amount of light reaching the sensor/film to record the image as your eye sees it. Getting that correct amount of light to the sensor is referred to as the correct exposure.

2. There are two variables you can manipulate to get the correct exposure.
a. The shutter speed. This tells you how long the sensor is exposed (uncovered). The longer it is exposed the more light reaches the sensor. You want the correct amount to reach the sensor. This is expressed in fractions of a second (i.e. 1/30 = 1/30th of a second).

b. The f/stop (iris diaphragm, aperture). This basically determines how large the opening in the lens is. If the opening is large, more light get through in a given period of time. If the opening is small, less light. This is measured as the f/stop (aperture) of the lens at that photo. The only thing weird about this is that the f/stop scale is inverted. An f/stop of f/2.8 is a larger opening, and f/stop of f/22 is a small opening.

It is worth noticing that the standard f/stop scale is f/2.8, f/3.2, f/4, f/5.6, f/6.3, f/8, f/11, f/13, f/16, f/22. There are other possible values. What the numbers mean is not important.
(Getting ahead of ourselves, notice that every third step is doubling. i.e. 2.8 x 2 = 5.6, 5.6 x 2 ~ 11, 11 x 2 = 22, or 3.2 x 2 ~ 6.3, 6.3 x 2 ~ 13. Each time you double the value, let's say from f/4 to f/8, you are letting in half as much light - remember f/8 is a smaller opening than f/4. This difference is referred to as "one stop" and we say that you have "stopped down one stop").

So you can manipulate both the time the sensor is exposed (shutter speed) and the opening of the iris diaphragm in the lens (aperture) to control how light gets to the sensor in order to provide enough exposure. So to get the exposure you need, you can either have the light coming through a large opening for a short period of time (e.g. f/2.8 for 1/500th of a second), or a small opening for a longer period of time (e.g. f/22 for 1/60th)...or somewhere in between (f/11 at 1/125th). They will all give you the same total amount of light and the same exposure.

If you are shooting in Aperture priority, you choose the aperture (f/stop) and the camera adjusts the shutter speed to allow the appropriate amount of light. So for example, if you did the change from f/4 to f/8 described above, you are letting in half as much light through the opening. So the camera will leave the shutter open twice as long (let's say 1/30th rather than 1/60th of a second).

If you are shooting in Shutter priority, you choose the shutter speed and the camera chooses an f/stop to allow in enough light during that shutter speed.

Why favor one (f/stop vs. shutter speed) over the other?

A fast shutter speed captures a moment in time. A snake's tongue flick may only last 1/10th of a second. If you use a slow shutter speed of 1 second, you will get a blurred image of the tongue since it moved during the shot. If you shoot at 1/1000th of a second, you can freeze that tongue and get a sharp image. But using a fast shutter speed means you have to open up the f/stop more to let more light in during that short shutter opening. (Sometimes we add flash to this type of shot to provide enough light for a fast shutter speed.)

F/stop determines how much depth of field you get. A small f/stop (f/2.8) only allows a small amount of the image to be in sharp focus. A large f/stop (f/22) allows a lot more of the image to be in focus. But a large f/stop requires a slower shutter speed so the camera/subject could move in that time.

So you have to make a judgement when taking a photo: How much depth of field do I want (f/stop) versus how much do I want to freeze motion (shutter speed)?

- Experiment number 1:
Put your camera on Shutter priority and take a photo of of a brightly lit room handholding your camera (no flash). Take a photo at 1/1000th of a second. Take a second photo at 1 second. Compare the two. The 1/1000th will be sharper because you held the camera still for 1/1000th of a second. It is hard to hold it that still for the full 1 second exposure.

- Experiment number 2:
Shoot two photos of the same scene/object. Shoot one at f/2.8 and the other at f/22. Compare the depth of field between the two (i.e. how much is in focus).

I can't believe I typed all of that out!!

Anyway, hope it helps....a little. It is just the beginnings of understanding the options you have with your camera, but it may be the most important one. There are lots of other reasons to make different choices but these are the fundamental two. The real key is that it is digital and film is free - shoot lots of pictures of furniture, your feet, dogs, trees, etc. to learn the ins and outs and then apply that to your herp photos.

Chris

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