technical question please

lynnma

Hi all, good morrow,

I have a Rebel 300D. (newbie) When setting my Fstop on AP of say 22 and letting shutter speed take care of it's self, to get the photo to expose correctly I'm compensating within the camera for snow of about 1 and 2/3 stops. My question is, where does this compensation take place, in the apeture setting? or the shutter speed. Also, what does the F stand for in fstop. This is the picture (ignore the composition) I'm just trying to get detail and snow and plant color correct. Also, the AP was 22 and shutter speed 1/6 (thats what it's telling me) before compensating, I'm assuming thats one sixth of a second right? Scuse lack of knowledge, trying real hard here...
Ta ever so.
Lynn

cletus

lynnma wrote:

Hi all, good morrow,

I have a Rebel 300D. (newbie) When setting my Fstop on AP of say 22 and letting shutter speed take care of it's self, to get the photo to expose correctly I'm compensating within the camera for snow of about 1 and 2/3 stops. My question is, where does this compensation take place, in the apeture setting? or the shutter speed. Also, what does the F stand for in fstop. This is the picture (ignore the composition) I'm just trying to get detail and snow and plant color correct. Also, the AP was 22 and shutter speed 1/6 (thats what it's telling me) before compensating, I'm assuming thats one sixth of a second right? Scuse lack of knowledge, trying real hard here...
Ta ever so.
Lynn

Lynn,

The answer to your first question - in aperture priority (AP) exposure compensation will adjust the shutter speed. It will leave the aperture alone.

I can't remember the answer to your second question about the origin of the "F" in F-Stop.

Third question - Yes, a shutter speed of 1/6 is one sixth of a second.

Hope this helps.

-Eric

lynnma

cletus wrote:

Lynn,

The answer to your first question - in aperture priority (AP) exposure compensation will adjust the shutter speed. It will leave the aperture alone.

I can't remember the answer to your second question about the origin of the "F" in F-Stop.

Third question - Yes, a shutter speed of 1/6 is one sixth of a second.

Hope this helps.

-Eric

Yep yep! thanks Eric - enquiring minds need to know these things you know.

cmr164

f-numbers

lynnma wrote:

Also, what does the F stand for in fstop.
Lynn

The 'F' in f-stop comes from f-number which derives from the ratio of the diameter of the lens to the focal length. F-stops represent points where there is a doubling of light and are set at powers of the squareroot of 2. Thus the stops at 1.4, 2, 2.8, 4.0, 5.6, 8.0, 11, 16, 22

2.0 = 1.4 * 1.4
2.8 = 1.4 * 1.4 * 1.4
4.0 = 1.4 * 1.4 * 1.4 * 1.4
5.6 = 1.4 * 1.4 * 1.4 * 1.4 * 1.4
...
11.2 = 1.4 * 1.4 * 1.4 * 1.4 * 1.4 * 1.4 * 1.4

And so forth.

You can get a pretty good idea of a lens' max f-number by just dividing the focal length by the front diameter of the lens. (just the glass). So a 50mm f1.8 lens should have a front element of 50/1.8=27.8mm and a 300mm f5.6 should have a front element of 300/5.6=53.6mm. Do remember that lens design is very complex these days and especially with zooms and ultrawides the lenses do not obtain the theoretical limit.

lynnma

cmr164 wrote:

The 'F' in f-stop comes from f-number which derives from the ratio of the diameter of the lens to the focal length. F-stops represent points where there is a doubling of light and are set at powers of the squareroot of 2. Thus the stops at 1.4, 2, 2.8, 4.0, 5.6, 8.0, 11, 16, 22

2.0 = 1.4 * 1.4
2.8 = 1.4 * 1.4 * 1.4
4.0 = 1.4 * 1.4 * 1.4 * 1.4
5.6 = 1.4 * 1.4 * 1.4 * 1.4 * 1.4
...
11.2 = 1.4 * 1.4 * 1.4 * 1.4 * 1.4 * 1.4 * 1.4

And so forth.

You can get a pretty good idea of a lens' max f-number by just dividing the focal length by the front diameter of the lens. (just the glass). So a 50mm f1.8 lens should have a front element of 50/1.8=27.8mm and a 300mm f5.6 should have a front element of 300/5.6=53.6mm. Do remember that lens design is very complex these days and especially with zooms and ultrawides the lenses do not obtain the theoretical limit.

Thanks Charles, I hope someone asks me what that means one day... I'll amaze them with my expertise. I have another question if I may. Am I right in assuming that because I'm taking pictures in snow (or some bright something) that the camera wants to underexpose so I'm compensating for that by adding in say 1 or 2 stops to let more light by the shutter? If I'm on Ap. Priority that is? I've been doing so much reading shot taking that my mind has gotton befuddled. I notice my picture looks a little "soft" I was expecting more crispness and I used a tripod.

It's so great to have this resource to learn from, I thank you.
Lynn

cletus

lynnma wrote:

...the camera wants to underexpose so I'm compensating for that by adding in say 1 or 2 stops to let more light by the shutter? If I'm on Ap. Priority that is?

That's exactly right!

pathfinder

lynnma wrote:

Thanks Charles, I hope someone asks me what that means one day... I'll amaze them with my expertise. I have another question if I may. Am I right in assuming that because I'm taking pictures in snow (or some bright something) that the camera wants to underexpose so I'm compensating for that by adding in say 1 or 2 stops to let more light by the shutter? If I'm on Ap. Priority that is? I've been doing so much reading shot taking that my mind has gotton befuddled. I notice my picture looks a little "soft" I was expecting more crispness and I used a tripod.

It's so great to have this resource to learn from, I thank you.
Lynn

Camera light meters assume that the world in front of the camera is a neutral 16% grey - so the camera wants to make snow look neutral grey - to make it look whiter and brighter takes about 1 or 2 stops more light - that is to say - you open your lens aperature from f8 to f5.6 or maybe f4.0 - or slow your shutter speed from say 1/250 to 1/125 or 1/60 - you increase your light exposure by one or two stops.
A good rule to know is that for a sunlit day with an ISO speed of say 200 - that the exposure for a normal grassy landscape is 1/ISO speed at f16 - so 1/200 at f16 for 200 IOS setting - open two stops for sunlit snow and bang 1/200 at f8 for ISO 200.

Are you using a camera that shows you a histogram? Like a 300D? If you are - look at the histogram and it will tell you what to do - Go to http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml for an excellent discussion of histograms usage by a previous professor of photography.

lynnma

pathfinder wrote:

Camera light meters assume that the world in front of the camera is a neutral 16% grey - so the camera wants to make snow look neutral grey - to make it look whiter and brighter takes about 1 or 2 stops more light - that is to say - you open your lens aperature from f8 to f5.6 or maybe f4.0 - or slow your shutter speed from say 1/250 to 1/125 or 1/60 - you increase your light exposure by one or two stops.
A good rule to know is that for a sunlit day with an ISO speed of say 200 - that the exposure for a normal grassy landscape is 1/ISO speed at f16 - so 1/200 at f16 for 200 IOS setting - open two stops for sunlit snow and bang 1/200 at f8 for ISO 200.

Are you using a camera that shows you a histogram? Like a 300D? If you are - look at the histogram and it will tell you what to do - Go to http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml for an excellent discussion of histograms usage by a previous professor of photography.

I've been shooting and ISO of 100.. for scenery etc.. would that be about correct? Obviously the higher the number the faster the film (or speed) but I'm looking for sensitivity right? So If I was going to take that shot it would be say f16 with a shutter speed of 1/100 open it up 2 stops for snow so it would be 1/100 at f8?
Lynn
Thank you thank you!! I'M OFF-READY TO TAKE THE PICTURE OF THE YEAR!

pathfinder

lynnma wrote:

I've been shooting and ISO of 100.. for scenery etc.. would that be about correct? Obviously the higher the number the faster the film (or speed) but I'm looking for sensitivity right? So If I was going to take that shot it would be say f16 with a shutter speed of 1/100 open it up 2 stops for snow so it would be 1/100 at f8?
Lynn
Thank you thank you!! I'M OFF-READY TO TAKE THE PICTURE OF THE YEAR!

1/ISO at f16 is an old estimate for sun lit exposure - ( per your example at ISO 100 == 1/100 at f8 for sunlit SNOW scenes ) This formula has been around as long as Kodachrome - 1930 or so? It is a good place to start if you do not have a light meter BUT you have something much BETTER than a light meter witha 300D!! You have a HISTOGRAM - and when you take a picture the 300D can be set up to display the histogram right along with the image fingernail displayed on the LCD on the back of the camera. The 300D ( Like my 10D ) comes set up from the factory to just display the image you captured - but in the menu commands is the ability to set the camera so it displays the histogram along with the image - this is a great thing! You can check your exposure immediately after your exposure and verify that it is correct. You want to see that the histogram is exposed almost all the way to the right for proper highlight exposure and that there is not too much of the highlights blown out They will blink where they are overexposed!!
Go to the luminous landscape link I posted above. M Reichman explains it all in much better detail than I can here. But once you understand and can use the histogram your exposures will improve dramatically


http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml