i know nothing about cameras & f ratios. i need some help. is this right. the higher the f # the smaller the field of view is & the shorter the wider. and what makes a shorter f# less exposure time . and my camera has 4x zoom when you zoom it w & t the f# goes 2.7 to 7.5. but in the manual mode you can move the f# without theeee zoom. does this mean i control the field of view at my choice of power.    example ; 18mm 169 power.f 2.6wider field than 169 power at f 7.5. and is the time of exposure shoter because a wider fiield draws light quicker. ive never took any photos of the sky so i dont know but is it a waist of time to take pictures of m42 and saturn with the moon at 3/4 full would it be to bright. this would be a focal w/nikon 4500. i know its a lot to ask. but any help i would be very thankful

                                   Andy & Terri

With cameras and their lenses, an iris diaphragm is used to control the amount of light that hits the film or chip. That is, the diaphragm opens or closes to varying amount, thus giving you one way to control the exposure. The other way is to vary the shutter speed.

Another factor controlled by the iris (aperture) is the depth of field.

The larger the F number, the smaller the diaphragm opening and the less light let through to the shutter and film or chip. Also, the larger the F number, the larger the depth of field.

The focal length and design of the lens determines the field of view. The F number has nothing to do with the field of view.

With telescopes, the F number has nothing to do with the amount of light coming through the scope because there is no iris diaphragm. The F number is simply the ratio of the scope's aperture and focal length.

If you have an 8" F5 telescope and a 12" F5 telescope, the 12" telescope's aperture is much larger and lets in more light. They both, however, have the same inherent F number and field of view. When you put an eyepiece into the equation, however, its design will determine the usable field of view at a given focal length.

Zoom camera lenses are like eyepieces in that they control the magnification and the field of view. A similar effect occurs with digital zooms in electronic cameras ... when you zoom "up" in power you gain magnification and lose field of view.

Don't confuse this will manually changing the aperture (F number). All that does is change the amount of light coming through to the film or chip, and change the depth of field (focus) of the shot.

Think of it this way: The F number is just a number on the camera dial to give you a reference point. While it's true it has a mathematical basis and is linked to exposure and depth of field as physical parameters, it really is nothing more than an indicator.

Nothing "draws light quicker". A larger aperture lets in more light (but at the same rate as a smaller aperture).

It is not a waste of time to attempt -- even to fail -- at taking astrophotographs at 3/4 Moon, as long as you learn something. If your target is very near the Moon in the sky at the time, you may get glare or internal light path reflections, however.

Andy n Terry, The one good thing about your 4500 which "looks" like a "pont and shoot" is that it has all the manual features of almost any SLR. This makes this camera a very rare breed. Try all the options you wish. The photo I showed you was in full auto mode, I believe this can be improved upon. The 4500 adjusts f ratio as focal length changes but you can change the F- ratio manually to a certain point being limited to the built in software parameters. These people know how far you can push their peticular lenz without exeeding the limitations too far even in manual mode, though you can still get a nasty result at extreems. I get junk everywhere myself with good stuff mixed in between. Im a rookie 4sure. Yes the time exposure can be shortened when the F ratio is wider(lower numerically). This is due to the maximum physical diameter opening of the apature being greater at lower f stop. f stop, yes it was f before it was F. In ceras is how far the shutter opens in relation to focal lengths, as in telescopes, but there is another factor. Shutter Speed. Seems like you already know though, the wider the apature opens the more light so less shutter time needed to gather light. Why cameras have the capability to adjust the f ratio at a given focal length has to do with the depth of field of focus. When you shorten the f ratio you can take a quicker ( faster shutter speed) pic but the cost is how much stuff near by your target object will come sharply resolved. If you take two photos of 30 cones placed 6" apart in line away from you one at f3 and one at f12, the f12 photo will resolve more cones than the f3. Sometimes it is more importaint to get all the faces in a crowd into focus. Other times as in objects near the highest end of focus, or infinity (far away from you, not is described in the 4500 or most other digitals) like overlook mountain views, this is not a factor so one may opt to use a lower f-ratio to shorten exposure time to be used as a tool to adjust for how much light is available. Under low lighting conditions like an overcast sky, at near infinity focus one can opt to shorten the f stop to allow more light into the lenz and take that photo quicker without longer shutter speeds. In Astrophotography in camera terms, all photos would be taken at infinity focus. You can pick your f ratio on your camera where ever it works best for your tracking system on your scope. The SCT on a EQ tracking abilities will allow longer time exposures than the nikon 4500 can do without overheating pixles. Try this. Use the Noise Reduction mode for dimmer objects. This will allow you to use the 8" and even the "BULB5M" settings best. It works better yet when you adjust the "Saturation Controll" to "Black&White" or maybe even "Sepia". Thats what I am going to try with a tracking platform. These get less noisy when in black and white mode. In full color mode which is "0 Saturation Control and no "Noise reduction" I can only last 13 secons shutter open time in Bulb before I see pixel noise. With Noise rduction on Ive done 2 minutes with very low noise. Much longer exposures possible in black and white and Noise reduction are possible. Also... As with all telephoto lenzes this 4500 lenz has a "sweet spot". In manual mode to get there push the button on the lower left of the viewing screen with the mountain,flower and clock images twice or till you see the flower appear at the top right of the viewer. THEN!!! Adjust the zoom with the W-T button slowly till the flower changes color from white to yellow.. This is slightly below the half way pont on the zoom scale that appears during this adjustment. At this focal length (or zoom)this Nicon lenz really stands up to its name. I really gotta hand it to Nikon for allowing us to find this area as I dont know of anyone else that does. All variable zooms have a best length BTW. would be stuck at a given image size though but... work that out later. Hope this helps, Ken

P.S. Please excuse the fact that all my latest posts have No indents or spaces between paragraphs. I have no Idea why this is happening as this is not the way I am writing my posts to this forum :( they just post this way. Ken

thank you, Jeff i'll print these replys. like i said i am new to all this and ive got a lot of learning to do. and i really didn't mean a waist of time, because any time you try is not a waist of time. what i should had said was.  with the moonlight could it be done. i was mad because i just got this 4500 and i was ready to go out this weekend and shoot, but i didn't know if i could with the moon like that. but you just made my weekend. well sun.& mon.anyway. rain friday & saturday. and ken yes i remember you moon shot. because i also have the 18mm scopetronix. i replyed back to see what size telescope you used. i have the 12'' sct & the 18mm is 169 power. i didn't know if that would be to much or not. if i could get a photo like yours i would be a happy man. any pointers please let me know.

                          Thank Ya'll Verry Much

                         Andy & Terri Byrd

Hi there, well here is what I know about f/numbers In my opinion the f/number (focal ratio) of any telescope tells two things about the telescope and that is it's intended purpose and it's photographic performance. The brightness of a star depends on the telescope's aperture not the telescope's f/number. All telescopes of the same aperture at any magnification will show the same visual brightness. There are many that insist their long focal ratio telescope gets higher contrast, this is not correct. A refracting telescope does have more contrast than any other because of it's optical system not because of the f/ratio. You can see that when you are comparing very well made and very well corrected refractors, you will see there is no gain in contrast regardless of the f/ratio of each telescope. Even if a reflecting telescope is well made and has the same size secondary mirror obstruction as another, it will have the same contrast regardless of the f/number of each telescope. All of the confusion and there is much on this issue, is because of the photographic use of the f/number. A faster f/ratio does mean brighter images on film but not in a telescope. Aperture not f/ratio is the important factor with a telescope. Some photographers have a great deal of trouble with this concept. The f/number of any objective lens or a mirror of a telescope has nothing to do with the visual brightness of an image, the bigger the aperture the better.