Lens/subject/format – an online simulator

How it works:

You enter the focal length of your lens, film or digital sensor dimensions and the approximate size of your subject. The simulator shows in value and graphically the following corresponding information:

1. the distance you must place your lens from the subject
2. the image circle and angle of view the lens must have to fill the frame with subject’s image
3. exposure compensation, if noteworthy, due to long lens movement
4. how much the lens must be displaced from focus at infinity.
5. distance from lens to image (film/sensor), important to assess bellows extension

The calculation is made in order to fill the frame proportionally. If subject proportions do not correspond to film/sensor then, to avoid cropping, side or top and bottom margins will show up, and a side or top view will be presented accordingly.

The simulator may be useful in cases like:

1. assess the distance from camera to subject in order to frame a picture with a certain couple lens and camera
2. assess and compare what can be done with different lenses/formats combinations
3. evaluate beforehand whether a lens will be able to focus a subject due to bellows restrictions, for being not short or long enough
4. project of a large format camera in which film/sensor, lens and bellow must be in tune with what kind of photography they are intended for
5. project of a photographic studio considering equipment, subjects and room size

Observations:

1. It doesn’t work with lenses that distort the image, like a fisheye lenses, for instance. The simulator uses formulas from optics that assume an ideal lens. The majority of commercial lenses are corrected enough to yield something reasonably close to that ideal in normal usage conditions. But in cases where technical limitations, or by wilful determination, the lens do not work corresponding subject’s proportions to image proportions, the simulator will no longer produce usable information.
2. Distances are calculated mathematically, and again, ideally. That may result in situations beyond equipment’s actual capabilities. For instance, in order to fill up a full frame camera (24 x 36 mm) with the image of a one Euro coin (∅~23 mm), a lens should be able to advance more that its own focal length (you can check using the simulator). For a 50 mm lens that would mean protruding more than 50 mm away from camera body. Practically no lens aimed to that format, film or digital, offers that possibility. That requires a macro-photography lens. A general purpose lens, either due to mechanical limitations or deterioration of image quality will be kept far from that extreme situation. The same applies to large format photography. Even being far more flexible than other camera categories, there is always a limit that the mathematical treatment, performed by the simulator, will not issue a warning about. It tells where the lens “should be” to take that picture. What the image circle and angle of view “should be” to comply with what you are asking for. Bottom line is that after getting a result, this simulator is useful exactly for allowing a check against specification of your equipment that will show whether the camera/lens can or cannot take that picture.
3. Photographic lenses are always a compromise; gaining in one side and losing on the other. As a rule of thumb, the more wide angle and/or the more bright a lens is, more the optical aberrations will take over and more the ideal formulas will fail to foresee the results. Do not expect a sharp precision with this simulator. But you can count on something very useful in average conditions, like individual or group portraits, urban, landscape, architecture, etc. In macro photography and all sort of technical work, the equipment becomes often so specific that the use of a general purpose simulator like this loses its attraction anyway.
4. For exposure compensation, this simulator does not consider any value below 1/3 of a stop. If simulator says that exposure must be increased by one stop and, for instance, the light meter indicates f8, move (opening) to the next stop, f5.6 in this case. Decimal figures mean intermediary positions that you can adjust by thirds or halves depending on whether your aperture dial moves continuously or clicks on those fractions. The need for exposure compensation comes from the fact that the apertures shown on lens ring are valid only for focus at infinity position. As the lens moves forward it spreads light in a larger area and its illumination level decreases.
5. The simulator was tested with many lenses and cameras, from small to large format. But, in case you find inconsistent results, please get in touch and we can try to figure out what is the solution for that.

References:

Only two formulas where used. The first one relates the focal length f, with distances u and v, from object to lens and from image to lens respectively. The second relates the same distances of u and v to image and object sizes, i and o, respectively as well.

For exposure increase, thinking of pictures in which lens moves far way from film/sensor, it was used exactly the relation between f and v, the focal distance to which the apertures were factory calculated and marked, and the actual distance from lens to film/sensor. The square of their ratio and logarithm are used to adjust scales and conventions.

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