I have some queries regarding the timings of the prayer.
1)As I came to know that in Hanafi jurisprudence the timings of Asr prayer starts when the shadow of the object becomes equal to twice the length of the object and in other schools of thought the Asr prayer starts with when the shadow of the object becomes equal to the length of the object. Which of the two is the correct timing of the prayer, say for example: my prayer timings shows that Asr prayer starts at 3:05pm while in Hanafi Masjid we see at 4:00 or 5:00.Which of the two is correct?
2) When does the timing of Isha prayer end?
3) When does the timing of Fajr prayer start? We find in Hanafi Masjids that when we finish our salaam for Fajr prayer the sunlight had already appeared but some knowledgeable people say that we should finish the Fajr prayer when the darkness prevails.
4) I have prayer time software downloaded from net. I find many options like the Umm Al Qura method, according to Karachi University, according to Al-Azhar University and options of various juristic methods etc. There are differences in prayer timings according to each choice. Which one is most correct? Please explain.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

The preponderant opinion of the people of knowledge in defining the timing of the ‘Asr (mid-afternoon) prayer is that it is when the shadow of an object is similar to the length of the object or a little bit longer than it.

As regards the timing of the ‘Ishaa’ (night) prayer, then it extends until midnight, although from midnight to the time of Fajr (dawn) is considered as a time of necessity for performing ‘Ishaa’, as it is not permissible to delay the prayer until this time without a valid excuse.

With regard to the timing of the Fajr prayer, then it starts when dawn breaks, that is, when the twilight starts and when complete darkness starts to disappear, until sunrise, as the Prophet, sallallaahu ‘alayhi wa sallam said: "Fajr prayer is valid until before the sun rises." [Muslim] However, the opinion of the majority of the scholars is that performing the Fajr prayer at the beginning of its time is preferable, although the Hanafi school of law is of the view that performing it when the yellow light spreads before sunrise is better for everyone except for a pilgrim in Muzdalifah, as it is better for him to perform it at the very beginning of dawn. Nevertheless, all the scholars agree that the Fajr prayer should not be delayed until sunrise; its fixed time ends at sunrise.

There is no harm in abiding by the prayer time-tables issued by the people of experience in the field as long as it does not contradict the signs set by the Sharee’ah, like the sunrise and sunset. However, we cannot determine the preponderant opinion among the sources that have been mentioned in the question as they are all trustworthy sources and the difference between them is not great. Therefore, there is no harm in taking one of these sources as a parameter to define the times of prayers as long as it does not contradict the juristic signs (related to their starting and ending time of each prayer) as already mentioned.

Allaah Knows best.