Sensor Size of the 645D (and More Technical Talks on DSLR Form Factors)

The standard lens of the Pentax 645 system used to be a 75mm lens, like the SMC Pentax A 645 75mm/2.8, which can be used on the original Pentax 645, 645N and the newer 645NII bodies.

To explain why 75 would be the ideal standard lens, the maths is actually easy. The dimensions of the 6 x 4.5 120 film size is simply 6 x 4.5cm, of which its name is self-explanatory, or 60 x 45mm. The diagonal length of such a film is thus 75mm, exactly.

In fact, I believe Pentax is the only manufacturer who has ever made a true standard lens for 135 film also, which is the FA 43/1.9 Limited, of which this focal is almost the diagonal of a 135 film frame of size 36 x 24mm (exact diagonal length is 43.2666 mm).

In fact, by looking at how compact the size and weight of both of the standard lenses 645 75 and the 43 Limited, we could know that such a standard lens is the easier lens to make for a properly designed interchangeable lens camera system, of which the back focus register distance (aka flange to film-plane or focal-plane distance) should be set as close to the standard focal length as calculated above as possible. That is, 43mm for the 135 system and 75mm for the 645 system.

In practice, the K-mount system was chosen to have a back register distance of 45.5mm, as seen in the summary table in the above link, whereas the EOS system is now being the one who has the closest to ideal standard focal value of 43.2666mm, amongst all current interchangeable systems, which is 44mm for the back focus register distance. The obsolete Fujica X-mount and Minolta MD are even closer, with a value of 43.5mm.

So, the rule for defining the back focus register distance is just that simple, for what I have already explained. So, for an APS-C system with a sensor roughly equal to 24 x 16mm, the ideal back focus register system should be 28.8444mm.

Now let's talk back some history. In 2003, in order to match better the smaller APS-C sensor size of Pentax's new *ist D body, which has a K-mount and old mirror house assemblies of Pentax film bodies, Pentax opted to create a new DA form factor in K-mount as their new digital lenses with a smaller image circle. The first DA lens was the DA 16-45/4, which was designed to maintain the old back focus register distance of 45.5mm of the classic K-mount spec. Whilst the true reason(s) for this "decision" is unknown, I must say this is just a very unfortunate thing, IMHO. Undoubtedly, they had made a very wrong decision, if they had really thought a bit more thoroughly before decided. Follow this article and you will know why..

Well, as a long time Pentax fans and supporter, I bought both the *ist D and DA 16-45 from the very first batch of goods when they were first marketed.

However, as I have explained about the rationale behind the design of the DSLR system form factor, the DA lens specification is just a compromise. As such, the DA lenses are forced to make larger as the optical formulas are complicated as a paid-off (not trade-off) for what the basic design rule is NOT followed. This explains why DA lenses, especially wide ones, cannot be made smaller but just larger and bulkier than all the film lens counterparts. For example, the DA 16-45/4 is larger and bulkier than the FA 24-90/3.5-4.5 (which has even longer focal range, but both effectively are starting from an effective AOV of 24mm wide), the DA 14/2.8 is larger and heavier than the FA 20/2.8, with a larger filter thread too.

Even worse, the optical performance of those corresponding DA lenses could not be better but worse, owing to the violation of the basic rule, again. Another good example is that it is almost well known that the newer DA 10-17 Fisheye by no means can match the superb corner to corner sharpness of the old FA 17-28 Fisheye of the past. The DA Fisheye is particular infamous for its pronounced CA and purple fringing at image corners.

Now, Pentax have announced the new DFA 645 55mm/2.8, which is supposed to be the new standard lens of the 645D, which has a smaller sensor than the size of 6 x 4.5 cm.

So, what's the new sensor size then? Suppose 55mm is the diagonal length of the new 645D sensor, the crop factor is hence 75mm/55mm, which is equal to 1.36363636..

Thus, the new sensor size can be calculated by dividing 6 and 4.5 cm with the above crop factor, which is exactly equal to 44 x 33mm. Or, this new DSLR should be called 4433D instead of 645D! ;-D

But yet again, with such a cropped frame without shortening the back focus distance, problems will arise, as what has happened for the DA lens system. But since now the crop factor is smaller than the 1.535 of the Pentax APS-C DSLRs, the adverse effect will not be as dominant.

Unless Pentax has made the DFA 645 55mm lens full frame, i.e., meant to be covering the whole 6 x 4.5cm image circle, it is meaningless to sacrifice. If it is "645 Full Frame", then it would be a better planning for future if some days later they can really make a 645 Full Frame body with a 6 x 4.5 sensor.

In fact, on the other hand, Canon was smarter to make the EF-S system with a shorter back register distance so that they could almost get the best of both world with backward compatibility with old EF lenses are ensured. Do note that the S in the name EF-S stands for "Short" (back focus register distance). They were indeed smarter than both Nikon and Pentax in this aspect. Practically, putting aside the better image quality and smaller/lighter lenses are made possible, the more important thing to the lens maker is that those new digital lenses are easier to make with lower costs, as the optical formulas and constructions can be simplified (and actually it is just the case that they have not been complicated).

Last but not least, the 645D sensor was once rumoured to use one of the Kodak large size 18MP sensor with size at around 48 x 36mm (I had the link of the datasheet but it is just a dead link now at the Kodak website). With this size, the standard lens should be 60mm then. But since Pentax is now telling us that the 645D will have 30MP+, I think they have sourced a new sensor, but the size would be different. Anyway, let's wait and see what the 645D will be when it really comes out! One to 1.5 years to come, probably..