关于摄影视场

使用35mm的胶片，要胶片完全爆光系统视场是否一定要达到44，使用数码相机呢？

没太明白这个问题。
视场 是以角度来表示的，与焦距有绝对关系，200mm焦距宽幅约为10度，400mm为5度。

数码相机使用等效焦距，结果一样。

用角度来表示的严格上来说应该叫视场角，而不是视场，只是大家叫随便了。视场应该是一个可视的区域范围，35mm的底片对角线为43.2mm，所以要让35mm的底片完全暴光成像，则“视场”的直径要大于43.2mm,所以是44mm。

请看以下文献：


A telescope having a large field of view is attractive, - 35mm Cameras, (or even larger), may be attached as well as being used for direct viewing, - but having a large field can have drawbacks.
Larger fields of view need bigger elliptical flats or secondary mirrors which in turn cause a larger obstruction to incoming light.
The "level of obstruction" is measured as the ratio of the obstruction diameter to the primary mirror diameter and expressed as a percentage, (D2/D1 in our Newtonian diagram). As an example, - a Newtonian with 500mm primary mirror and an elliptical flat of 100mm, (minor axis), would have an obstruction ratio of 20%. The drawback is that Obstruction ratios above say 20-25% begin to cause a noticeable deterioration of the contrast ratio.
What this means is that if the obstruction ratio is likely to be above say 25%, - then the designer should try and aim for the smallest elliptical flat or secondary mirror that gives the field required.
Other types of telescope and/or medium format photography may involve large fields of view and often push up the obstruction ratio to over 40%. In this area the designer must make the secondary mirrors and elliptical flats as small as possible while still achieving the field required.
So how Big a field do you need? - It depends on what you are going to do with the Telescope.
If you are only going to use the telescope for direct viewing with an eyepiece, then one recognised authority states that a 10mm field is quite sufficient. However other authorities suggest 14mm. Neither of these figures are likely to give you a large obstruction figure with a simple Newtonian telescope, - so you might decide to provide a larger than strictly necessary elliptical flat? - which gives a bigger field, - while still keeping the obstruction ratio less than say 25%
It gets more difficult if you want to take pictures with a 35mm Camera. The diagonal of a 35mm negative is about 43.2mm, so a field of say 44mm is needed to cover the full area of the negative. Depending on your prospective telescope's size's, - designing for this size of field may give you an obstruction ratio above 30%, - in which case you should always aim for the minimum sized flat necessary.
(If you are contemplating Astrophotography, - then please see the page on this website dealing with the subject.)
If you do need a big field and cannot avoid an obstruction ratio above 25% - what does a poor contrast Ratio actually mean?
It means that while you will be able to take very good views and good pictures of stars, - which have a very high contrast ratio to start with, - you will not do quite as well with the moon or planets as these objects have poor contrast to start with.
As the obstruction ratio increases the fine detail on low contrast subjects like the Moon and planets will begin to fade. This fading increases as the obstruction ratio increases.
(However - please remember that if you just want to take pictures of stars, - then don't worry about obstruction ratio at all! - a ratio of over 50% will still give good pictures on stars.)
Note Cassegrains with wide fields often have obstruction ratios of 40% or over, and no-one says they are bad telescopes. You just have to live with the obstruction ratio that goes with the field you require.
Although you really need a field of 44mm for full illumination of a 35mm negative, - there is room for compromise. The object you are aiming at is usually deliberately positioned in the centre of the picture, and you are usually not as interested as to what is at the edges? One recognised authority suggests you can still achieve acceptable pictures with the lighting level at the edge down to 70% of the centre.
Since the light level does not drop off immediately to zero once out of the fully illuminated area, there is a considerable distance available before the light level drops to 70%. For an average Newtonian of say F/3 to F/8, a fully illuminated field of 32mm will still be giving 70% or better at 44mm.
If you are especially interested in photography and a more even light level over the entire negative is very important to you, - then you should consider increasing the fully illuminated field from 32mm to some value closer to 44mm; - 36mm will give about 85%, and 40mm about 95%.
It's really up to you what compromise between size of fully illuminated field and obstruction ratio you decide to go for. Remember that a field of 32-44 mm can be used for both 35mm Camera photography and direct viewing, - but a field of 14mm, while excellent for direct viewing and possessing a very low obstruction ratio cannot be used with a 35mm Camera.
For larger Newtonian Telescopes over say 12" Diameter, - even a 44mm field usually has an obstruction ratio less than 25%, - so even if your principle use is expected to be direct viewing, - you might decide to design these for a 32-44mm field so they can accept a 35mm Camera at sometime in the future?
If you are considering the use of a CCD Camera, - You must find out the diagonal size of the sensor used in the camera and use that as the field of view required. Note that CCD's are usually fairly small and a field of 14mm for direct viewing is sufficient to cater for most CCD Cameras, - but check first.

没错。

数码相机，CCD一般都不外露，前面的镜头不可拆卸，并且大部分是变焦的，前面还要加目镜，因此不能一概而论。但总体对于望远镜的视场要求很低。

单反数码与胶片类似，现在有全幅的，但更多的不是。例如Nikon D100，要求视场直径为28.4mm就可以无暗角。